Does the School of Necromancy feature Command Undead have a usage limit? The feature does not specify the number of times I can use it, all it says is
As an action, you can choose one undead that you can see within 60 feet of you.
This feature does not seem to use spell slots or state the number of times I can use it in a day.
Does this mean I can use this skill at will?
Or I can only use this in combat because it requires an action?
If none of these are correct please clarify.
It doesn't have a usage limit, but It does have a control limit on the number of undead.
This effectively limits your Command Undead to One Undead type creature, letting you change your focus to another Undead if you so desire, but losing control over the first one in the process.
This means you can use it again on any other number of Undead creatures, provided you keep in mind the requirement to only have One under command at any one time.
In general, if a feature uses a spell slot or has any other distinct requirements they will be mentioned in the description as explicitly as possible.