Your Microchip sales rep can give good answers to that. Typically, for a small, mature chip like the ATTiny85 they can deliver a million immediately and quickly

Your Microchip sales rep can give good answers to that. Typically, for a small, mature chip like the ATTiny85 they can deliver a million immediately and quickly ramp up to any volume you need in a few months. But there are many similar chips and one may be much more available than the next. Definitely build several prototypes first to see if people want it before investing a lot of time in building a supply chain.