Often I get asked in classes what the limits are for SharePoint. Like how many files can I have in a document library and how many folders, how many lists in a site etc.
Microsoft published a really nice whitepaper on technet covering loads of limits, from how many user profiles per farm to how many items in a list.
One thing to keep in mind though: IT IS ALL RELATIVE
Having 50000 items in a document might be quite feasible on the right farm, but if you're using your SQL Server for other stuff and he is running at max speed anyway (150+ Transactions per second for example) you will feel the pain faster than when he's got 4 quad cores, 20GB of Ram and is generally quite bored (10-50 Transactions per second)
Another good post I managed to scrouge of the internet is on the limit of columns in a list. The above article claims you can have up to 256 different field types in a list and up to 4000 columns. But can you? If you ever come across the column limit exceeded error message or your columns are not behaving the way they should (filtering not working properly for example) then you have reached the field type limit.
These were the settings for WSS 2.0 and SPS 2003:
64 Single line of text and Choice (drop-down menu or radio buttons)
31 Multiple lines of text and Choice (check boxes (allow multiple sections))
32 Number and currency
16 Date and time
in WSS 2.0 these limits were set and they were transferred to WSS 3.0 Although they are not enforced the same way no more and you can end up creating more than those in some cases, expect the system to start acting up and behaving badly when exceeding those limits.