When you pay for extort do you have to pay for each card separately or jus pay one mana for all the cards on the field
Each instance seperately
You pay B/W for each trigger of extort.
Example 1: you have 2 creatures with extort and you cast 1 spell.
You may pay up to total of 2 mana for 2 triggers of extort.
Example 2: you have 1 creature with extort and you cast 2 spells.
You may pay up to total of 2 mana for 2 triggers of extort.
Example 3: you have 2 creatures with extort, which one of them is {Pontiff of Blight}. You cast 1 spell.
You may pay up to total of 3 mana for 3 triggers of extort. In this case, pontiff gave your other creature a total of 2 abilities of extort, hence 3 triggers in total.
Note all mana mentions meant B/W.