Like Kumirei pointed out, I’m maintaining a fork of this project as Towe is retiring theirs. If you have new questions, feel totally free to ask on the thread for my fork since I’ll be monitoring for alerts on it to answer new questions.
The quick answer though is: the numbers count the amount of items left at each SRS level for the current review session. There are 4 apprentice levels, 2 guru levels, 1 master level, and 1 enlightened level.