You do have SOME control, it just comes with the disadvantage of extra lag from the supply chain. If you need 50k extra wafers to catch up with demand, you can still get them, you just have to wait six months for your extra order to reach the front of queue if you didn't already have an option for additional wafers or a sufficiently large one in the original agreement to get them sooner.
And hopefully the others are not putting in more to keep ahead of the curve and pushing yours out further. Yes order extra but it will be a bit of a struggle.
Its not the end of the world but if AMD really wants to compete with Intel they need to be able to fill vendor channels at the drop of a hat and they have already had some supply issues with the higher end chips. The more popular they get in mainstream, HEDT and server markets the more this will become an issue especially as more and more companies move to 7nm.
This also means they would make sure certain channels are filled more than others. Mainstream is not as much of a money maker as server/HPC is so that's where I imagine they would focus most of their available supply.
I am honestly surprised Intel didn't run into supply issues long ago considering they held 99% of the server market and 80% of the desktop market for many years.