A pack is 10 reports for $9, paid once. There is no subscription, nothing renews, and the reports do not expire — they sit on your account until you use them.
Buy a pack if you are weighing up an idea or two and then getting on with building. Take Pro if you are working on this every month: it is far cheaper per report, and it opens the whole list on every category and search-term page, which a pack does not.
Packs and Pro are not exclusive. If you are on Pro and buy a pack, the pack waits until your monthly allowance is spent — your own allowance is always used first, so buying one early never costs you anything.
You can buy another pack whenever you run out. There is no limit on how many.