My invention can work in general if it has 2 or more compartments. Can I just make it as general as possible and say "2 or more compartments", or must I claim a specific number?
For a broad independent claim "two or more compartments" seems perfect if it is the case that it works with any number of compartments above 1. You could also say “at least two compartments”.
If there is a sweet spot of number of compartments, say four, for some efficiency or symmetry in manufacturing or use, it can be a good idea to have a dependent claim reciting "where the at least two compartments are exactly four compartments".