Is there a rule about buying more units than you have models?

I've been looking through the rulebook and I don't see anything about it, but maybe this is just one of those "there's no rule saying dogs can't play basketball" moments.

Let's say your army composition is set up where you've spent the minerals to have two units of Stalkers, but you only own the models for one unit of Stalkers. Could you deploy the one unit, then when they die you deploy the second unit?