[smackfu]

It totally depends on your prescription. If there’s no astigmatism, the lenses can be stocked and cut to the frames you need. Astigmatism adds another two variables that make it much harder to stock. Also the stronger your prescription, the more alternative material options there are to keep the thickness down, which again makes it harder to stock everything.

[UtilityDave]

Most offices who keep stock lenses keep cyl (astigmatic correction) up to two diopters in quarter increments in my experience.

The main limitation is usually material choice.