How many Sonic games are there?

There are around 40 Sonic games, ranging through the Sega consoles to mobile and arcade versions, the most knowledgeable ones are.

Sonic the Hedgehog 1, 2, 3 and 4.

Sonic and Knuckles.

Sonic Spinball

Sonic 3D Flickies Island

Sonic Spinball

Dr. Robotnik's Mean Bean Machine

And a few others that can be found on Sonic Mega Collection Plus.

Sonic CD

Sonic the Fighters

Sonic R

Again, more can be found in Sonic Gems Collection

Sonic Shuffle

Sonic Adventure 1 and 2, including the DX and Battle additions.

Sonic Heroes

Shadow the Hedgehog

Sonic the Hedgehog 2006

Sonic Unleashed

Sonic Colours

Sonic Riders

Sonic Riders: Zero Gravity

Sonic Free Riders

Sonic Advance 1, 2 and 3

Sonic Rush

Sonic Rush Adventure

Sonic Battle

Sonic Rivals

Sonic Rivals 2

Sonic Chronicles the Dark Brotherhood

Mario and Sonic at the Olympic Games

Mario and Sonic at the Olympic Winter Games

Sonic and the Secret Rings

Sonic and the Black Knight

Various arcade games.

And many more that aren't mentioned here. ( Including the online games.)