This setting defines maximum number of automaticly saved games (autosaves) in \OpenTTD\save\autosave\ directory.
Manual can automatically save the game every set period of time. Naturally, more saved games means much more disk space used. This setting limits the number of saved games.
If all savegame slots are already used, older savegames will be overwritten (oldest first).
This setting is not used if Keep all autosaves is used.
The value is set to 16 by default. You can change it by using the following syntax in Openttd.cfg:
[patches] max_autosave_num = value