You only need one girl to replace the previous one. If you keep more than one, then your herd size increases (which you usually don't want).

Well I guess if you're at maximum capacity you wouldn't want additional female calves, but I would imagine some of those could be sold.

I have a friend who had a beef cow farm (which is slightly different, but close enough). Basically, you goal in building a herd is to strategically breed the best cows that you can. When you start, you buy some cows. Depending on what you are doing, you may or may not keep a bull yourself (you might artificially inseminate the cows, or pay for the services of a bull). Generally speaking you inbreed the cows, looking to develop specific traits that you want and occasionally breed your cows with a bull that you don't own for specific purposes (essentially improving the blood line). Once you have your initial cows, you almost never buy another one -- you just pay to breed with a higher quality bull. There are obvious exceptions, but that's the general rule.

It takes time to breed a herd of cattle. My friend had an outbreak of foot and mouth and he lost his whole herd one time. It took years and years and years to build it back up. At best you can double your herd size every year. In practice, though, some cows don't produce a calf every year. Also, there are cows that you don't want because they don't have the traits you want. But once you have the herd size you want, it is a steady state thing.

I tried to find some place to find the volume of cattle traded that's not destined for slaughter, but the data doesn't exist. It seems that dairy cows are traded mostly via want ads on the internet these days. Here's a typical site that specializes in it: https://www.dairylivestockservices.com.au/stock-for-sale/ Notice how all of the livestock is fully grown. That's because these are livestock from failed farms. I don't think anybody sells calves because there just isn't a market for it. You can't sell calves for meat either, because as I mentioned earlier, it's not economical (the price for a meat calf going to slaughter is only $125!)

It is sad to think about it, but the reality is that milk production necessarily relies on the death of cows to stay economical. If we drank a lot less milk and were willing to pay maybe 10x the price for it, then you could change that fairly easily. However, it's just not possible at anywhere near the price point we have for food right now.