Minecraft – the villager breed rate

You will need to return at least 2 villagers. Then they will start repopulating the village. As of 1.2, villagers will mate and repopulate based on the number of available houses. See this post (especially the "your village died out" section) for more information.

As of the 1.8 snapshots, though, the villagers need to be "willing" to breed in order to breed. It seems that you can make a villager "willing" by trading with them. Willingness is granted on the first new trade offer, and at a 1 in 5 chance per subsequent trade. Once a villager is willing, they will immediately seek another willing mate, if the village can sustain more villagers (based on the number of houses).

If villagers are emitting hearts, then you've done all you could for them. Now it is only a matter of them entering love mode simultaneously, which is entirely in the hands of God of Random.

I've been frustrated with my villagers not mating once, too. You can't do anything, regrettably. Just ensure your villagers can meet easily once they want to and wait - possibly find yourself something to do nearby.