How to implement repeatable group

[BMBurstein]

I couldn't find how to do repeatable arguments in the documentation, and I need something more complex than that.
How do I accept only pairs of strings? So any of these would be OK:
my_cli_app str1 str2
my_cli_app str1 str2 foo bar
my_cli_app str1 str2 foo bar a b c d e f

but not these:
my_cli_app
my_cli_app str1
my_cli_app str1 str2 foo