well i am making like a towering game, and i was wondering what would be the easiest way to make enemies come more and more based on how long you play. I.e there will be 5 people coming than the next, time 7 people, than the next time 9 than 11 than 15 than 20....

I know i could do a time line but making a timeline for every single wave is kind of hard. any suggestions?

Just make it so they spawn more and more until you kill all monsters. For starters, go 3, 6, 10, 15, and so on.
(Random Quadratic Statement off the top of my head)