Postgres - autovacuum on several big tables
If only one database needs any attention, then upto autovacuum_max_workers will all get launched in that one database.  But it will take a few passages of autovacuum_naptime to get all of them launched.  Each one will take a different table to work on at a given time.  

This might not have the effect you want, however, as the IO throttling threshold will get divided up between them.  That means that while multiple tables are vacuumed in parallel, increasing the number autovacuum_max_workers does not increase the overall speed of vacuuming when you have IO throttling turned on.  The purpose of using a higher value of autovacuum_max_workers is to ensure that small tables don't get entirely neglected if all the (would-be-lower max number) of workers are individually pinned down by large tables.

If you are worried autovac is not working fast enough, I would just go for the big guns and set autovacuum_vacuum_cost_delay = 0 in order to turn off IO throttling, and set log_autovacuum_min_duration = 0 so you can see the what is going on.  This should quickly give you a definitive answer.  As opposed to incrementally changing autovacuum_vacuum_cost_limit, which will slowly give you ambiguous answers.