As you can see I copied the characters that were available and repeated their pattern to see how many character slots can actually fit this character select screen. There are 22 slots that can fit, meaning that there is room for two more unrevealed characters. However, I do believe that if Superbot wanted to, they could shorten, or completely remove the star boxes, making room for more characters.
On the vita there is only room for the current 20 plus the random button
actually theres also 22...i just noticed, add two more next to sly cooper, than go down from those two all the way to the left, if you count its 22 also. theres room to put two more to the right