Just want to correct this idea which has been repeated several times on this topic: You only need 3 or 4 for a larger restaurant. The entire restaurant does not pay simultaneously.
When you (the server) are done with them you just put them back in a charging dock or some other communal space.
It makes a lot of sense, in my view: Handling orders tickets digitally is probably incredibl useful for the kitchen (especially considering a mix of dine-in and takeout orders).
From there, it‘s only a small (ok, not really small because PCI-DSS, but at least possible) step to also having these things do payments.
[0]: The European Banking Authority (EBA) calls on traders to make use of the exemption for strong customer authentication up to 50 € as allowed under Commission Delegated Regulation (EU) 2018/389.
For this reason alone, you do need a PIN pad at every terminal.
Credit cards have no PIN in the US (and debit cards can usually be used without one as well), which makes this approach possible in the first place, and the tipping culture that can make it desirable to get a quiet moment with the check to figure out amounts.
Practically, it also means that even larger restaurants usually only need very few POS terminals at a central location, rather than one per waiter, requiring wireless infrastructure in larger restaurants etc.
This is changing very fast, though – I‘m seeing both combined order and payment handheld devices (that support tap-to-pay, i.e. also Apple Pay) as well as the QR payment solution in more and more places.