Receptacle size for Stand up freezer
We don’t go by start up amps we go by what is called FLA full load amperage the 21 amps is locked rotor current, when motors start they draw 3-5 times the normal running value, that’s ok because it is usually less than 1 second so no heat has time to build up.
If the unit came with a plug that is the normal 2 parallel and a ground or the standard 15 amp 120v plug you can use a 15 amp circuit. If it is 1 blade rotated 90 degrees 120v it is a 20 amp plug and needs a 20 amp circuit.
Code has exceptions for motor circuits that require higher starting currents than normal the limit is 225%  NEC 440.22.
So there are exceptions and we work with the information on the nameplate. It will usually recommend a circuit size of 15 amps with a maximum of 10 amps.
Who wait you said the starting current was 21 amps! Well the circuit breaker is an inverse time device the higher the current the faster it trips. A 15 amp circuit will normally hold at 50 amps for a half second (you only need 21). So the amperage peak is very short and your current is already on its way back to FLA or 2 amps.
Putting a plug in device like this on a 30 amp circuit would be a code violation. And I would bet it will be fine on a 15 amp circuit.