What size turkey should I get for 18 people?
Are you channeling your best self with this comment? (If you're not sure, check out our Code of Conduct.)
Get three small ones and cut them in half. Check this out:
The general rule of thumb is a pound per person, but maybe go larger so you'll have leftovers. 24-26 lbs ought to cover it.
I would do two separate turkeys. The really large ones never seem to come out as tender and evenly cooked.