Is it safe to charge iPhone 16 Pro Max with a 20w adapter and 240 USB C charging cable?

Just to add a bit to Zachyy's excellent reply. Don't let the "240" rating for the USB C charging cable mislead you. It doesn't mean that it provides 240W of power; it only means that it can handle that much ...

... but even if you had a 240W power adapter and this cable, as Zachyy has mentioned, the iPhone will control how much power it will draw from that adapter.

Yes, that is perfectly fine. Your iPhone will only draw out the necessary power it needs.