At the beginning you only have 10 Sudokus but you can unlock more Sudokus until 50 completing the different sudokus.

Sudokus	How to unlock
Sudokus 11 to 20	Solve 5 of 10 sudokus.
Sudokus 21 to 30	Solve 10 of 20 sudokus.
Sudokus 31 to 40	Solve 15 of 30 sudokus.
Sudokus 41 to 50	Solve 20 of 40 sudokus.