Yes, there are years they run out of visas. This is really only an issue towards the end of the fiscal year (they spread out issuance in a usual year). We did have some fairly recent cases, I think it was two years ago (sm1smom might confirm or correct the year) where some people here interviewing near the end of the FY were told there were no visa numbers available. As mentioned above, they cannot reserve visa numbers in advance for AOS like they do for CP when a consular interview is scheduled.