I’m new to draft and/or sealed and can’t remember how many basic lands to play – how many should I have in a 40 card deck?
You should usually play 17 lands the vast majority of the time.
If you have a lot of expensive spells (5+ mana cost) or need to reliably hit land drops 1-5 on time, then you should play 18.
If you have a very low mana curve (no 5+ cost cards, few 4s), then play 16. On the rare occasion all of your cards are 3 mana or less, you may be able to play 15.