One subscription, both products.
Nim is free to use. Reading a page costs nothing at all: the dictionary for Chinese, English and Spanish lives in your browser, so looking a word up is instant and unmetered however much you read.
Pro raises the daily limit on the things that need a model to answer:
One subscription covers both the app and the extension. Sign in with the same email in each and Pro follows you.
Cancel whenever you like. Prices include local tax where it applies. Questions: the address in the privacy policy.