An extra space after a template ID parameter will cause an error when adding multiple template IDs. Example:
What to do
Remove the spaces and it will fix the error. If you are using formula fields/if statements, make sure to add the comma before the ID in the clause.
There is a limit of 10 template IDs allowed. You will receive an error message you exceed this amount that says: You have exceeded the maximum allowed templates per merge. Allowed: 10
