Specifies the maximum number of autovacuum processes (other than the autovacuum launcher) that may be running at any one time. The default is three. This parameter can only be set at server start.
If you have an installation with many tables (100's to 1000's) or with some tables which autovacuum takes hours to process, you may want to add additional autovacuum workers so that multiple tables can be vacuumed at once. Be conservative, though, as each autovacuum worker will utilize a separate CPU core, memory and I/O.