Can Circle of Protection:White prevent the damage from Leeches (Circle of Protection / Leeches Combo)?
The current Gatherer text for Leeches is the following:


  Target player loses all poison counters. Leeches deals that much damage to that player.


Circle of Protection White:


  {1}: The next time a white source of your choice would deal damage to you this turn, prevent that damage.


When you cast the spell, there is only one copy on the stack. All the damage is dealt at once. You only need to activate the CoP:White once.

Leeches is a white source of damage by 609.7a and thus may be prevented by CoP:W


  609.7a If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that's waiting to apply, or by a delayed triggered ability that's waiting to trigger (even if that object is no longer in the zone it used to be in); or, for certain casual variant games, a face-up card in the command zone. A source doesn't need to be capable of dealing damage to be a legal choice. The source is chosen when the effect is created. If the player chooses a permanent, the effect will apply to the next damage dealt by that permanent, regardless of whether it's combat damage or damage dealt as the result of a spell or ability. If the player chooses a permanent spell, the effect will apply to any damage dealt by that spell and any damage dealt by the permanent that spell becomes when it resolves.