i assume this is a question about iOS, I don't think android nor Windows have developer device limits.
Apple states that you may register up to 100 of each device type per year, so it looks like 100 iPads, 100 iPhones, and 100 iPods. That's a change from the past, where you could register up to 100 devices, period.
I had not actually noticed this policy change but I assume it is in effect. For years there were rumors of Apple increasing the device limit but it never happened.