According to Google's current environment you can add up to 30 aliases or alternate emails for each user.
Sign in to your Google Admin console using your administrator account (does not end in @gmail.com).
From dashboard » go to Users.
» Click a user's name
» Click the Account section.
» Scroll to the Aliases section. Then click Add an alias.
» Enter an alternate user name (the part of the address that appears before the @ sign).
Note: You can't create an alias currently assigned to someone else, either as a primary address or alias.
If multiple domains have been added in your Admin
» Click the dropdown to the right of the @ sign. A list available domains are shown. Select a domain for the alias address.
For more info click here.