Can I trade in my iPhone 15 Pro Max for a 17 Pro Max?

Look here -

https://www.apple.com/shop/trade-in?afid=p240%7Cgo~cmp-21676437520~adg-166399978745~ad-739794424286_kwd-9885952940~dev-m~ext-~prd-~mca-~nt-search&cid=aos-us-kwgo-brand-account-launch-040825-

Yes, you can get a trade in allowance, provided the 15 was bought in your country and is in working undamaged condition.

See the Trade in section for the amount when you order the phone in Apple.com or take the phone to the store.