Main Load Center for subpanel
Don't  scrimp on panels

One thing we constantly preach here is to get plenty of extra spaces.  Oh, you can install a 4-space panel, hook up your lights, saw, dust collector and general sockets, mission accomplished... And then boom, wife wants an EV.  There's a planer but it's 240v.  The inspector comes by and says your 4 circuits need AFCI, but you used double-stuffs.  Changing panels is a nightmare, and the cost difference to a much larger panel is a few latte's.  Maybe a pizza.  So we say "go big or go home". Nobody ever complained about too many extra spaces. 

(And by the way, panels that say "24 spaces 48 circuits", that's a lie. These days Code will almost never let you use the double-stuff breakers needed to get to 48, so that panel is effectively 24.)

An outbuilding requires a main shutoff switch

A panel with a main breaker is just a cheap, compact way to provide that shutoff switch.  You can also use an accessory shutoff switch that is in a separate enclosure. 

Don't bother trying to coordinate the trip rating of this "shutoff switch" breaker with the actual supply breaker in the main panel. That must be sized to protect the wire to the subpanel.  For the breaker in the subpanel, size doesn't matter. If it is too big, that's fine. If it is too small, it protects the subpanel itself.

Trying to get the local "main" breaker to trip first is a lost cause, unless you use a shunt breaker and some sort of local smart equipment that will fire the shunt trip on a more restrictive -- gahhh, that would be hard to build.

You may be able to make this panel have a main breaker.

Some panels allow Backfeeding -- that's where you fit a breaker with a special tiedown kit, and feed the power into the screws where you normally take the load.  Since it's AC power, the breaker doesn't actually care which way power flows. 

Traditionally one puts a backfeed main breaker in the upper left corner.