Regular Expression(RegEx) can be used for multiple Emails separated by coma validation rule in Salesforce.

Syntax:

OR(   
	NOT(   
		REGEX( 
			Emails__c , 
			"([a-zA-Z0-9._-]+@[a-zA-Z]+.[a-zA-Z,]{2,4})*"
		)   
	),   
	RIGHT( Emails__c , 1 ) = ","   
)