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Wow, you must have a really big space to be starting with that many birds! Congratulations and good luck with your hatch!
Yes, always beware of free chickens... seems like they're usually cockerels! Often times the feed stores will offer specials on straight run (unsexed chicks).
Depending on how many birds you plan to have in your flock, usually a single "good" rooster would be fine for approximately 10 hens, maybe a 2nd as a backup. Having more than one can possibly lead to excessive fighting, but usually they will just end up dividing your flock into separate "harems" as it's more peaceful that way. So just be sure you enough space and housing for them all to coexist.
Here are some excellent articles to make sure your coop(s) and runs will be in good working order:
How Much Room Do Chickens Need
Chicken Coop Ventilation - Go Out There And Cut More Holes In Your Coop!
Predator Proofing
Oh, and one more article just for fun:
Keeping A Rooster
View attachment 1608321 Best wishes in your new chicken adventure!