It is a one time purchase - like a microwave, dishwasher, car, etc - you purchase once and it works for as long as you have it. Unlike the others, you get free updates for the first year you own it, and updates after that first year are considerably cheaper than purchasing it again.
I’ve never understood why people believe a one time purchase fee for software should enable them free access to any additional development, when nothing else in the world works this way.