The quantity of the Allied troops was fine. The quality was not. Britain and France had been largely unprepared for war, and the training of their conscript armies was abysmal. In Britain, ammunition shortages had the notorious result of each recruit being allowed only five rounds in total for rifle training. The French conscripts were more badly trained still. Fortunately, the small British Expeditionary Force had many professional troops rather than recent conscripts.
By contrast, the Germans side had had much more intensive and elaborate training. Accurate, full-scale mockups of crucial fortifications were built in Germany, and troops rehearsed their attacks until perfect.
Their first great mistake was relying on the Maginot line, the highly fortified border between France and Germany. In the first world war it would have been impregnable. In the second world war, concentrated attack eventually breached it, but more significantly, the Germans attacked first through Belgium and Holland instead, rendering it not just worthless but a handicap, as it required large numbers of French troops to occupy it.
The second great mistake was the attempted defense of Holland and Belgium. Relying on neutrality to protect them, the Low Countries had not heavily fortified their frontiers, and did not allow Allied troops to enter, even for reconnaissance, until actually invaded. "He who defends everything, defends nothing" was Frederick the Great's famous saying, but this concept was ignored. British and French troops abandoned their defenses on the French-Belgian borders and advanced into Belgium, with the intention of resisting the German Panzer divisions in unfortified, open, unfamiliar territory.
Thus the scene was set for the great disaster. Poorly trained troops, many of whom had fired only five shots in their lives, commanded by officers using obsolete tactics, stationed in inadequate defenses in unfamiliar territory; prepared to defend themselves against the combined firepower of the Panzer divisions and the Luftwaffe.
With the benefit of hindsight, it's always easy to look back and declare authoritatively what should have been done, especially since your ideas will never actually be tested by the enemy. In the case of the Battle of France, because the armored assault tactics were new, it was not clear how to defend against them.
In 1940 it was not clear that air superiority would be such a decisive factor in a ground conflict. It was expected that strategic bombing would be more effective than it was. It was not known that urban areas could provide effective resistance to tank assaults, as happened in Stalingrad. The critical weakness of paratroop attacks, that they are vulnerable during and immediately after their descent, was not certain. The importance of fortifying positions all around, rather than relying on a line that faces one way only, was underestimated. The potential of radio communications, and the inability of traditional couriers to cope with rapidly moving fronts, were not accepted.
When faced with the first effective use of new tactics by an enemy, it is easy to be appalled in hindsight that they were not anticipated. In practice, it takes time and experience to develop and deploy defenses. In the Second World War neither Poland, Belgium, Holland, France nor Russia, were at first able to resist the combination of armored divisions and air superiority. With fewer mistakes, the German attack could have been made more costly for them, and the battle more protracted. However, without the crucial factor of experience, it is doubtful that the final outcome of the Battle of France could ever have been different.
On the 22nd of June France surrendered to the Germans. Fighting continued for a few days before dying out. German casualties were only 27,000 dead, 100,000 wounded. French military casualties about 100,000 dead, 200,000 wounded. Worse was to come for the French: about 400,000 civilians would die in bombings and in forced-labor camps under the German occupation, and another 100,000 military would die during and after the liberation. In the cruelest cut of all, 1,147 French sailors would be killed by their British allies at Mers-El-Kebir on the 3rd of July, when the British decided they would destroy the French fleet themselves, rather than risk it falling into German hands.
Armour:
In May 1940 France had over 3,000 tanks, and in terms of numbers, quality and firepower they were generally superior to those employed by the Germans. Tactically, however, the French were definitely inferior, and with one or two exceptions the handling of this valuable force was uninspired and wasteful. The basic armoured unit was the tank battalion, comprising a staff, three tank companies and a reserve company (in all, some 45 to 60 tanks). There were 39 tank battalions in 1940 and for tactical purposes they were twinned to form tank battalion groups. At the outbreak of war only five battalions had received the latest tanks (the Somua S-35 and Char B-1 models) while the rest still retained a variety of modern and semi-obsolete types. In addition to the battalion groups there were also 11 independent tank companies.
Between January and May 1940, four reserve armoured divisions were formed and placed at the disposal of the General Headquarters. These divisions absorbed a number of the above-mentioned tank battalions and companies. Armoured fighting vehicles were also to be found in the cavalry divisions undergoing mechanisation, in five light cavalry divisions which were also still not completely mechanised, and in the reconnaissance groups. All these formations combined horsed cavalry, motor-cycle troops, armoured cars, tanks, and infantry mounted in cross-country vehicles.
French Air Force
The Air Force had suffered considerable neglect between the wars, and its performance in 1940, with machines which were in general much inferior to those of the enemy, was not impressive. Indeed, according to French sources, only 420 modern fighters and 31 heavy bombers were serviceable when the Germans struck in May.
Total French front line strength on 10 May 1940 was 1,604 aircraft (764 fighters, 260 bombers, 180 reconaissance planes, 400 liaison aircrafts).
Total strenght of the French Air Force together with Aviation Colonaile: 1,200 fighters, 800 reconaissance planes, 1,300 bombers.