Sumo Digital’s Steve Lycett stated the below about Sonic & All-Stars Racing Transformed:

The thing is, different people think of different things when you say ‘How many characters are in the game’.
Are we talking about playable choices, drivers, vehicle forms, which platform are we talking about, what?

Me, I go with actual playable choices on the character select. Over all the different platforms, this totals up to 29 actual playable choices. But that doesn’t mean that there are 29 playable choices in all platforms (in fact no single platform has all 29). Some drop down to the low 20′s, some instead have high 20′s.

Why is this? Well some platforms have characters that are unique to that platform. I’m not going to say which, that’s part of the fun of the reveals! Some are SEGA characters, some are not. Don’t bother asking :p

Different versions of Sonic & All-Stars Racing Transformed on different game systems will have different rosters of characters to play as. Some characters might be “unique to that platform,” so maybe the Nintendo versions will have some Nintendo exclusive characters? I can’t really see Mario making an appearance though.

Source: gonintendo.com