It's not a formal context, it's very obviously written in a breezy and casual style, where attention-grabbing is more important than formalities

The call-response format is short-long in stressing, which gives it a PUNCH that "fewer" would lose as well

1 2 3 4 5 6 1 2 3 4 5 6

Feels more punchy than

1 2 3 4 5 6 7 1 2 3 4 5 6 7

Also, it's not a recruitment poster, it's a political advocacy poster.