It will apply the replenishment buff to everyone in the raid regardless of the raid size, I'm not currently logged in but I believe that the tooltip even says so. Hope this clears your query up.
It only effects 10 people at once. It stays on the target for 15 seconds, but every time you cast it it refreshes. So, in the beginning if you cast 1 on your tank, it hits 10 players. 2nd, on 10 other players. It is smartcast, so goes on the lowest mana player.
So basically, if you are in a 25 and the only person that gives replen, after you have 3 LBs up when you refresh or cast LB again, only once, only 10 people at a time will have it.