I can't fully speak for the Fastmail folks, but I know that there are a few upper limits for how many masked email addresses that one account can create. We tried to set them unreasonably high to allow for all manner of legitimate use while still preventing bad actors. They're also monitoring usage and tuning that limit. Plus, you can always email support and ask for a increase for your specific account, if you ever bump up against it.
I can't fully speak for the Fastmail folks, but I know that there are a few upper limits for how many masked email addresses that one account can create. We tried to set them unreasonably high to allow for all manner of legitimate use while still preventing bad actors. They're also monitoring usage and tuning that limit. Plus, you can always email support and ask for a increase for your specific account, if you ever bump up against it.