Renpy has no built-in maximum number of save slots. The default code generator that runs when you create a new project does output screen code that limits the number of slot-pages you can quick-jump to.
Looking at the code for Renpy 8.x:
This default "file_slots" screen that is used by both save and load screens is created with 10 pages of 6 slots, plus auto and quicksave.
See "screen file_slots(title):" in the file screens.rpy for details.
The 1-10 pages count is arbitrary but also controls how many "save page numbers" are attempted to be shown in the "prev/auto/quick/{page number links]/next" links row on the save/load screens.
It's trivial to increase the page number count, but at some point the developer will end up having to change the page numbers list display to handle the increased numbers.
Even without changing the displayed count, you can use the "next" button to go past page 10. The quick jump links will not exist however beyond page 10.
Looking specifically at the code for save/load screen in TTBS:
This is based on an older version of Renpy: v7.4.10.2178
The save/load screen appears to have been customized. (The original version shipping in that renpy version can be reviewed here:
You must be registered to see the links
. This is clearly quite different to the one in the game code.)
In the screens.rpy file, we can find the custom screen "filepicker", and in here there is the "limit" of 25 pages set in the parameter to the FilePageNext function.
If you want to have more pages, increase the "25" to whatever you like - but remember the only way to see pages > 25 is by repeatedly clicking the ">" button.