Is there any kind of ratio for how much water/food you need per settler?

i think so, i think it is 1 food for every settler and 1 water for every settler, i had 5 foods and 2 things with 3 water each and 5 settlers

You need exactly 1 food : 1 bed : 1 water - per existing settler to support them.

And food + water produced = defense value needed to reduce raids

Can not prove, but believe having extra surplus helps in how fast you attract new settlers via beacon.