There's a 50 dose limit, set by the Customs and Border Protection Agency when bringing drugs back into the US, without a US prescription. There are some exemptions for bringing prescriptions in from Canada and Mexico.
The FDA has its own rules, set at a three month supply, requiring a prescription, and the drug must be legal in the US. You'll see conflicting reports on whether you can get through customs with a three month prescription.
For example here, from the FDA:
"Travel with no more than you need for your personal use during your stay. A rule of thumb: Bring no more than a 90-day supply of medication."
https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm484154.h...
Does that apply only to foreigners, to US citizens, both? Who knows, their own information across agencies seems conflicting and confused. They're probably all unsure of which agency actually has final authority at this point, and or they each probably think their agency does.
That said, every year millions of Americans bring in illegal prescriptions regardless. If you're desperate enough and the savings is high enough, why not try it within reason (stopping short of some large cache that gets you tagged for smuggling that is).