Every plan gives you a pool of usage each month. Everything a video needs comes out of that one pool: the picture, the motion, the music and any sharper finish. You don't pay per video, and your plan price stays the same however you use it.
Some videos use more than others, so how much one uses can vary. Longer videos use more than shorter ones. Motion uses more than a still scene. Finishing at 1440p or 4K uses more than 1080p. Shorter videos with a still scene are the lightest on your usage.
Re-rolls only use usage for the part you remake. Redo the music and you use usage for the music, not for the whole video again.
If a render fails, it doesn't use your usage. If you cancel a render partway through, the parts that had already started still count, and nothing after that does.
You can always see what's left at Settings → Usage: how much of your monthly usage is left, when it resets, and any extra usage in your packs. If a render needs more than you have left, it won't start, and Studio tells you before anything runs.
When your monthly usage runs out, usage packs top you up, as long as Extra usage is switched on at Settings → Plan & billing. Buy extra usage with usage packs has the details, and How usage works covers the monthly pool on each plan.
If something doesn't add up, email [email protected] and a human will read it.