I currently board my 2 horses.
6 summers months it is $100 a month per horse.
6 winter months it is an additional $60 a month per horse for hay
(They buy the hay but I know that my 2 horses will eat a round bale in about 2 weeks and round bales can cost anywhere from $50 apiece to $100 if there's drought conditions, and I'd need about 10 to get through an average winter typically.)
So for 2 horses, hay and board are over $3,000 for the year.
Farrier I see every 5 weeks to keep on top of Red's problem. About 8 months of the year he has on shoes so for the two of them it's about $130 every 5 weeks, and then in the winter (no shoes) it's $70 each time.
So for the year for 2 horses, the farrier is about $1,300 a year.
They get their spring check up every year with teeth floats, sheath cleaned, vacinations, fecal, and dewormer supply. I always have "extra" things due to Red's problems, but those things probably run around $500 or so.
They see the chiropractor a few times a year, so that's roughly $400 for those visits ($60 per horse per treatment).
For me, grain is minimal as they are easy keepers and don't need much. So I maybe spend $100 on that a year.
I choose to feed a very good supplement. That runs me around $300 (or more) for the year for the 2 horses.
So for 2 horses for me, for what I consider BASIC things, we're looking at over $5,000 a year.
If I had 4 horses, then it'd be closer to $10,000. Granted, I have some board money in there, but that could easily be "used up" on routine fencing supplies, paint, etc and usual repairs that you will need to do as part of owning your own place. And that's not setting aside any money for emergency vet bills!
Plus, if you are starting a new place, you are going to have a lot in start up costs to put in your fencing, run in sheds, a barn, corrals, and whatever else you need.
So not trying to discourage you, but I would start with only 2 horses (so they have a buddy) and try to OVER-estimate what your costs and expenses are going to be. You are smart to start planning now.