You will always have access to the latest version. It's a one-time payment.
Really? The FAQ says this :
"This page has a unique URL for your purchase. You can always re-download the latest files from this page. You should have also received an email with a link to this page. If you ever lose the URL for this page you can request it to be
emailed to you from the support page." So I think you need to use the same URL to download the latest versions.
Also, here! https://itch.io/docs/creators/how-buying-works I am 100% sure you don't need to buy every version seperately!