Plans differ along a few axes. Exact allowances and prices are on the pricing page, which reflects live configuration — anything written here would go stale.
What varies
Which to pick
If you are evaluating, the free allowance is enough to model an organisation and run real tasks — enough to judge output quality.
The reason to move up is usually parallelism and depth rather than volume: more agents per task is what changes how hard a question you can ask.
Changing plan
Upgrade or cancel from Billing. Cancelling leaves you active until the end of the period you have paid for.