A multi-select field is not a real database field. Internally a multi-select field is implemented using one associate table and two database relationships. For more details see: Multi-Select Lookups
This means that there is no simple "Required" flag.
Require at least X Selections
There is two different ways to require a selection from a Multi-Select.
1) Use a Table Trigger to validate
One option is to use a table trigger and throw an error if there is not enough child selections.
2) Using a Stored Procedure to validate
A Store-Procedure is used to validate and save the record. The Procedure can perform any necessary validation and either raise an error (causing a roll-back) or it can fail to advance the status and set or return an error message.
This scenario is typical when the form is setup to be a Wizard. For more details see: Data Entry Wizard