What you are describing is known as the modulo operator (usually represented with the percentage symbol). Consider this example skip logic:
sys_RespNum() % 5 = 1
The part left of the equal symbol will divide the respondent number by the number 5 and give you the remainder of that division. Then the skip will fire if that remainder is equal to 1.
That being said, there may still be better options. Modulo will give you an even number of people sent into each group, but will not ensure you have an even number of completes in each group come analysis. "Even Distribution Quota" from the Community Question Library attempts to dynamically fill out groups by sending respondents into whichever group has the fewest completes. You can learn more about it here: