Samara had three daughters, their "father" was another asari. Not sure about the differences in the daughters' age.
I would guess that they can have multiple children with the same partner. I know Shepard isn't an expert on the asari but in the romance scene with Liara after LotSB, Shepard mentions the idea of "a lot of litlle blue children". I would think it weird if they couldn't have more than 1 child per partner but who knows.