Looking at the source:
The score is held in an unsigned long, which could contain a maximum value of 4294967295. I don't see anything in the game that limits the maximum score, so presumably it could go that high before wrapping.
The score converted to a string using ltoa() and then displayed using the contents and length of resulting string. So it should be able to display the maximum value of an unsigned long.
Therefore, the maximum score before wrapping is 4,294,967,295.
However, @JO3RI, note that there is a bug in the drawNumbers() function. buf is only 10 bytes long but a ltoa() conversion could result in 11 bytes: 10 digits plus the null terminator. So, (however unlikely) the buffer could overflow if the score goes above 999,999,999.