Orsdxy

I have a single ethernet switch. Should I use spanning tree?

If there is a redundant link, definitely need to enable STP.
In a single switch, how is it possible to have a redundant link?
Any other possible scenarios?

To add to Zac67's and JFL's answers:

In case you decide to enable spanning-tree on the single switch, don't forget to configure the client's and server's switchports as Edge Ports (Cisco speak: spanning-tree portfast [trunk] or spanning-tree port type egde [trunk]). This avoids having the Spanning Tree Listening and Learning phases on the switchport, after line protocol has come 'up'.

DHCP client software (and other things, too) on the end systems is usually not very happy with the 30 seconds of silence after - from their point of view - the NIC's line protocol has come up. Usually, clients and servers and the software they run are not aware that it the takes the (portfast-less) switchport 30 seconds to go into forwarding mode.

In short: A sanely done STP configuration - even on a single switch - is a good safety net for your network. A badly executed STP configuration however (classic examples: Bridge priorities not set, "wild" topology with various/random link speeds, forgot portfast) can be worse than none.

STP protects your network in case two switch ports are connected together, so you should use it generally.

With a single switch, you can have redundant links to an appropriately configured host. However, you must not connect multiple links between switches/bridges unless you're using STP, LAG or SPB.

As explained by Zac67 answer STP is normally only useful when connecting several switches together.

However there's other related features than can be useful on a standalone switch.

BPDU Guard will protect your network in case 2 links are connected to the same device. The most common case in my experience is with IP phones.

BPDU Guard will disable the port if the port receive a BPDU on a port on which it is not expected.

Of course to use this feature, (x)STP must be enabled.

So yes you may benefit from activating STP to use BPDU Guard, if there's a risk of loop - especially if you use 2 ports IP phones.

Edit

Also you should enable spanning-tree portfast or equivalent, see Marc 'netztier' Luethi answer for more details