How many clubs have played in the premiership?

It's 42.

In addition to the 20 teams that are currently there, these teams have also graced the top division since 1992: Barnsley, Birmingham, Bradford, Charlton, Coventry, Crystal Palace, Derby, Ipswich, Leeds, Leicester, MK Dons (as Wimbledon), Norwich, Nottm Forest, Oldham, QPR, Reading, Sheff Utd, Sheff Wed, Southampton, Swindon, Watford and Wolves.