Changing my Minecraft username

Do I have infinite Minecraft username changes? Or is there a specific number of changes (if yes, how many?)? After a specific amount of usernames, would I have to pay for a new one?

As this help page (archive) states, you can change your name every 30 days.

Your old name is free to be used by someone else after 37 days.
Fun fact: To try out all possible names, you would need 5139697974392368644520926551 years. The fact that some names (archive) are owned by Mojang and guaranteed to always work the same in the future (for player heads) and the fact that about 91 million usernames are already taken barely changes that number.

There are unlimited amount of times you can change your name for free.

You can only change your name once every 30 days.

If you've changed your username already, you'll need to wait 30 days to be able to make another change.

If you change your username, your old name is held for 37 days. This gives you an extra week to change back if you've made a mistake or changed your mind before your previous username becomes available to anyone else.

Source: Mojang