Basic thumb rule for number of temporary databases is minimum of two (one for sa and DBA activities) and another for non-sa end users. This allows sa/DBAs to connect even when user tempdb is full.
If applications/SQL are really well written to share the tempdb in a responsible cooperative way you do not need more than 2 tempdbs.
Beyond that if a set of users/applications want to run a free-for-all in their own user tempdb then it is just like a scratch/work database for them. And the number of tempdbs can be as many as demanding users/applications you have with as many as your hardware can support.
Except recovery, some lazy/delayed writes and automatic housekeeping, SAP/Sybase treats tempdb just like any other database but with a flexible user security.
HTH
Avinash