IIRC, on the 4th September fighter-command only had "50" Spits left, not much.
Spitfire numbers did drop to about 50 on the 4th September. But that's the number in the reserve, which went from about 70 in late August to 40 something on the 7th September.
The front line squadrons had well over 200 Spitfires, with the returns for 4th September showing 218 Spitfires serviceable, along with 407 Hurricanes, 50 Blenheims, 21 Defiants and 8 Gladiators.
At no point were the RAF reserves in storage exhausted, and front line strength remained at 100%, with losses being made good from the reserve.
Another of those graphs, from the same source:
That's one of the things that worried the RAF, reserves were decling, which meant, in time, front line strength would begin to decline as well.
What the RAF didn't realise was the Germans had very few reserves, had exhausted them long before, and had suffered a major drop in front line strength.
Bungay references Milch's report on his tour of Luftwaffe airfields in France in late August/early September. He reports that the gruppen were seriously under strength, with gruppes supposed to have 35 - 40 aircraft, the bomber gruppes averaged 20, the 109 gruppes only 18.
So whilst the RAF reserves were declining, they were never completely exhausted, and front line strength never declined, whereas the Luftwaffe had no reserves, had very low aircraft production, and were well below strength.
The RAF BoB website has the serviceable figures in their daily reports:
http://www.raf.mod.uk/bob1940/calendar.html
They don't give them for the first few days, but:
17 July
* Blenheim - 67
* Spitfire - 237
* Hurricane - 331
* Defiant - 20
* Total - 659
24 July
* Blenheim - 56
* Spitfire - 238
* Hurricane - 294
* Defiant - 15
* Total - 603
31 July
* Blenheim - 63
* Spitfire - 239
* Hurricane - 348
* Defiant - 25
* Total - 675
7 August
* Blenheim - 66
* Spitfire - 256
* Hurricane - 368
* Defiant - 24
* Total - 714
14 August
* Blenheim - 59
* Spitfire - 219
* Hurricane - 342
* Defiant - 25
* Gladiator - 2
* Total - 647
21 August
* Blenheim - 58
* Spitire - 239
* Hurricane - 400
* Defiant - 25
* Gladiator - 7
* Total - 729
28 August
* Blenheim - 55
* Spitfire - 225
* Hurricane - 413
* Defiant - 23
* Gladiator - 7
* Total - 723
4 September
* Blenheim - 50
* Spitfire - 218
* Hurricane - 407
* Defiant - 21
* Gladiator - 8
* Total - 704
11 September
* Blenheim - 61
* Spitfire - 214
* Hurricane - 387
* Defiant - 21
* Gladiator - 8
* Total - 691
The figures bounce around a lot, because they are serviceable aircraft, not aircraft on hand, and after a heavy period of fighting the serviceable numbers go down temporarily. But the RAF actually gained front line strength during the battle, the number of Spitfire and Hurricanes squadrons gradually increased, from 44 on the 14th July to 51 on the 1st September.
Richard Overy, The Battle, sums up in one sentence why the BoB is often represented as a very close run thing, with the RAF only narrowly staving off defeat in September. He goes into a description of how the British (and Americans) greatly overestimated the size of the Luftwaffe, and German production capability, and how the Germans underestimated the size of the RAF, and British production capability. Then:
The British fought the battle as if it were a last ditch struggle against an overwhelming enemy; the German side fought against a force persistently misrepresented as technically and tactically inept, short of aircraft, pilots and bases.
From a British perspective, they were losing the BoB. Their losses were unsustainable, and in a matter of months they would be reduced to a state where they couldn't defend British airspace.
The Germans felt they were winning. Their losses were high, but they thought the RAF's losses were much higher. Their strength was declining, but they thought the RAF's strength was declining faster. They knew they couldn't keep the same level of attack up for weeks, but they felt the RAF was only days from collapsing.
What the sides believed then still colours accounts, because it comes through in all the official records of the time. It's also a good story, and Churchill made some memorable speeches about it.
But from an impartial perspective, with full access to both sides losses and replacements, it's clear the RAF were winning from the start. They actually increased strength for the first months, where the Germans declined. They declined in strength slightly in late August and early September, but the Luftwaffe declined sharply at the same time, and expended all their effort in attacks that the RAF weathered fairly comfortably.
Here's another graph, showing the production balance for both sides:
For the RAF there's only a brief period when losses outstrip replacements. For the Luftwaffe losses outstrip replacements for almost the entire battle.
As to how successful Op. Seelöwe would have been "if" the British airforce had given up, well I'd guess pretty successful.
I don't think Sea Lion would have been, because the Luftwaffe cannot stop the RN sinking the invasion fleet, but if the RAF had been defeated, then unhindered German bombing might well have been sufficient to force Britain out of the war.