You'll probably be limited more by the connector on a single fan than anything else, but you should try and keep the total for fans/pumps under 4A or so. The fans are fed from the merging of the four 12V rails through some 5A low-drop diodes. You'll likely get some current sharing between the diodes, but due to the way diodes work at high temperatures it's best to assume a worst case that all the current would go through a single one of the diodes. 50W of cooling should be enough for any normal situation.
If you want to hook up six 250cfm screamers in push-pull across a triple radiator, pumped by a
1/3HP 3300GPH sump pump, you should hook that up direct to the power supply.

Also, I want pictures.