anti-affinity: what're the mechanics?

Hey, all. Turns out we really need anti-affinity running on our (very freaking old -- Juno) clouds. I'm trying to find docs that describe its functionality, and am failing. If I enable it, and (say) have 10 hypervisors, and 12 VMs to fire off, what happens when VM #11 goes to fire? Does it fail, or does the scheduler just continue to at least *try* to maintain as few possible on each hypervisor? Thanks! -Ken