What determines how many rules an Exchange Server mailbox can have?
An ancient Messaging API (MAPI) design decision stating that a folder's properties list can't exceed 32KB in size is responsible for limiting the total amount of rule data that Exchange can store. So, in theory, you can have thirty-two 1KB rules, one 32KB rule, or anything in between. Determining how this 32KB limitation affects the number of rules you can have is difficult because rule sizes vary according to the number of conditions and exceptions that you apply. This MAPI design limitation has been consistent from Exchange Server 4.0 through Exchange Server 2003, and I don't expect it will change in future Exchange versions.