At least 3

Looks like there are 3 baby heads that poke up when the parents come with food. (though the picture only shows one) Because the babies are still so small the parents don't have to come very often with food so sometimes they are out singing while the babies are sleeping. Nothing is allowed in your yard (other than humans)- everything else gets chased off by the robins.
There still seems to be an egg in the nest as the parent was poking at it.