We dont know exact numbers, but Boon has given a solid idea. He has, on multiple occassions said "Similar size to MK9" and "Thirty Something"
MK9 with DLC had what? 31?
DLC is already confirmed to be two guests and two Klassics. Goro is another +1, but another classic.
So we have (Using Commanders list as a rough outline)
14 Returning Characters, 4 New.
We would need 7 more new characters to make it the "Most new characters ever". So essentially we would need every character after this to be new and bring back almost none of the characters from MK4-Armageddon to make that possible. So unless the roster has balloned to nearly 40 characters, it is entirely unlikely.
1. Sub-Zero
2. Scorpion
3. Raiden
4. Kano
5. Sonya (Special Forces faction leak / reveal, if true)
6. Jax
7. Mileena (Challenge Tower rumor)
8. Kitana
9. Kung Lao
10. Reptile
11. Quan Chi
12. Ermac
13. Ferra / Torr
14. Kotal Kahn
15. D'Vorah
16. Cassie Cage
17. possible Johnny Cage (since Sonya is in)
18. possible Liu Kang (Challenge Tower rumor)
19. MK9 character
20. MK4 thru MKA debut character
21. MK4 thru MKA debut character
22. MK4 thru MKA debut character
23. MK4 thru MKA debut character
24. MKX
25. MKX - Unnamed bow staff guy
26. MKX
27. MKX
28. MKX
DLC:
29. Goro
30. Klassic Kombatant (according to the "Kombat Pack")
31. Klassic Kombatant (according to the "Kombat Pack")
32. Guest (according to the "Kombat Pack")
33. Guest (according to the "Kombat Pack")