db2fw is db2 fast writer. These are used for event monitors to write high volume event records efficiently. These run in parallel and each writer can service more than one event monitor.
This architecture was introduced in db2 9.7 to decouple the application agent from the event monitor writing, plus of course, the efficiency of parallel writes.
The number of db2fw is based on the number of CPU cores. If you find too many of them, then you can reduce the number by using a registry variable.
Contact db2 support if you need this variable, as I do not see it documented in public domain.
Cheers
Sathyaram