From GPWA Wiki
With traditionally high player values and real growth opportunities, marketing online casinos is a popular choice for affiliates. Between popular slots games, video poker and table games, the online casinos these affiliate programs represent have something for every player. And many of them boast massive jackpots to pique player interest. Most affiliate programs are willing to work with webmasters on creating bonuses or other special promotions for their players.
According to Online Casino City there are over 750 online casinos in operation.
According to iGamingAffiliatePrograms.com there are about 280 affiliate programs representing online casinos.
About 65 of the affiliate programs that represent online casinos are GPWA sponsors.
There are about 3,500 sites listed in the GPWA member portal directory that promote online casinos.