From Online Manual
SMF was written with hard-coded limits on some of the things stored in the database. These were set based on assumptions about typical database and webserver capabilities as well as some assumptions about typical forum communities. If your forum is much bigger than usual, you may need increase these hard-coded limits. When you do that, you should be aware that you may need to investigate optimizing operations involving the quantities you are now increasing beyond what the developers planned for. Enjoy!
- Attachment -- 2,147,483,647 (while theoretically 4,294,967,295 is the upper limit on some configurations)
- Category -- 255 (but may have issues beyond 127)
- Events (Calendar) -- 65,535
- Members -- 16,777,215 (but may have issues with more than 8,388,607)
- Posts (all Topics and replies) -- 2,147,483,647 (while theoretically 4,294,967,295 is the upper limit on some configurations)
- Topics -- 16,777,215 (but may have issues with more than 8,388,607)
- Themes -- 255 installed. You may have trouble administering Themes and Layout with more than 100 themes installed.
Note that these values include both current and old values; if you delete a category it still counts towards the limit. Limits may also be dependent on server software.