I'm not an expert by any means but there are a very large number of variants:
Main bomber type with Jumo 211 engines.
Ju 88 A-0
Pre-production aircraft.
Ju 88 A-1
Initial production variant. 895 kW (1,200 hp) Jumo 211B-1 engines
Ju 88 A-2
Jumo 211 G-1 engines.
Ju 88 A-3
Conversion trainer. Dual controls.
Ju 88 A-4
Improved variant. Long span wings. Modified with new bomb dropping equipment to produce a A-15 "special" bomber variant. RLM refused to authorise mass production, as the wooden bomb bay "bulge" caused too much drag and a thus a reduction in speed.
Ju 88B
Prototype with all-new fully glazed "stepless" crew compartment nose, developed into Ju 188.
Ju 88 B-0
10 pre-production aircraft with "stepless" fully glazed nose.
Ju 88C
Zerstörer, fighter-bomber and night fighter, based on A-series, but with sheet metal nose.
Ju 88 C-1
Planned fighter variant, powered by two BMW 801MA engines. Never built.
Ju 88 C-2
Initial production variant.
Ju 88 C-4
Heavy fighter, reconnaissance variant.
Ju 88 C-5
Improved heavy fighter variant.
Ju 88 C-6a
Improved Ju 88 C-5 variant.
Ju 88 C-6b
Night fighter variant.
Ju 88 C-6c
Night fighter variant.
Ju 88 C-7a
Intruder variant.
Ju 88 C-7b
Intruder variant.
Ju 88 C-7c
Heavy fighter variant.
Ju 88D
Long-range photo-reconnaissance variants, based on the Ju 88 A-4.
Ju 88 D-1
Long-range photo-reconnaissance variant based on Ju 88 A-4.
Ju 88 D-2
Long-range photo-reconnaissance variant based on Ju 88 A-5.
Ju 88 D-3
Tropicalized long-range photo-reconnaissance variant based on Ju 88 A-4.
Ju 88 D-4
Tropicalized long-range photo-reconnaissance variant based on Ju 88 A-5.
Ju 88 D-5
Ju 88G
Night fighter, new fuselage with A-series' ventral Bola gondola omitted, tail section from Ju 188, optional Schräge Musik.
Ju 88H
Long-range photo-reconnaissance, fighter variants, based on the stretched Ju 88G-series fuselage.
Ju 88 H-1
Long-range maritime reconnaissance variant.
Ju 88 H-2
Fighter variant.
Ju 88 H-3
Long-range maritime reconnaissance variant.
Ju 88 H-4
Fighters variant.
Ju 88P
Anti-tank and anti-bomber variant with single Bordkanone series 75 mm (2.95 in), 50 mm (2 in), or twin 37 mm (1.46 in) calibre cannon in ventral fuselage gun pod mount, which mandated removal of the Bola gondola under the cockpit section, conversion of A-series bomber. Produced in small series only, they were perceived as a failure for both anti-tank and anti-bomber use.
Ju 88 P-1
Heavy-gun variant fitted with single 75 mm (2.95 in) Bordkanone BK 75 cannon in ventral gun pod. Appeared in mid-1942 in small numbers.
Ju 88 P-2
Heavy-gun variant with twin 37 mm (1.46 in) Bordkanone BK 37 cannon in ventral gun pod.
Ju 88 P-3
Heavy-gun variant with twin 37 mm (1.46 in) Bordkanone BK 37 cannon in ventral gun pod, and additional armor.
Ju 88 P-4
Heavy-gun variant with single 50 mm (2 in) Bordkanone BK 5 cannon in ventral gun pod. There were 32 built.
Ju 88R
C-series night fighters with BMW 801 engines.
Ju 88S
High-speed bomber series based on Ju 88 A-4 but with ventral Bola gondola omitted, smoothly-glazed nose and GM-1 nitrous-oxide boost, fastest of all variants.
Ju 88 S-0
Fitted with two BMW 801 G-2 engines, single 13 mm (.51 in) dorsal gun and 14 SD65 (65 kg/143 lb) bombs.
Ju 88 S-1
Fitted with two BMW 801 G-2 engines, the GM-1 boost system and could carry two SD1000 1,000 kg (2,200 lb) bombs externally.
Ju 88 S-2
Fitted with two turbocharged BMW 801J engines, wooden bomb bay extension as used on the Ju 88 A-15.
Ju 88 S-3
Fitted with two 1,671 kW (2,240 hp) Juma 213A engines and GM-1 boost system.
Ju 88T
Three-seat photo-reconnaissance version of S-series.
Ju 88 T-1
Based on the Ju 88 S-1 but with bomb bays fitted for extra fuel of GM-1 tanks.
Ju 88 T-3
Based on the Ju 88 S-3.
In particular where is your interest? I'm building the A-4 at present