Laurie A. describes how sleep overs generally work for my granddaughter too. I'll add that there are no sleep overs on a school night. Sleep overs are more often during the summer but still not more than 1-2/month. A sleep over is usually combined with some other activity such as plans to go swimming or skating the afternoon before or the next day.
I don't have room for a sleep over at my house and since my daughter works I do provide a fair amount of recreation. I take my granddaughter, who will soon be 10 and her friends swimming, skating, to play in the park, to a movie at least once a week in the summer. Then they sleep over at my daughters house sometimes but often they don't.
What guides us is whether or not it is doable from my daughter's or my point of view. I think it does help for us to share the time between us.
Play dates are more common. They can last just an hour.
We aren't concerned about reciprocation. I believe that some families have the ability to have sleep overs or play dates and some don't. I can't honestly tell you if my granddaughter has been as often to her friends' homes as they've been to hers.
The number of girls to have depends on how well they get along and how much energy I have. I do firmly believe that best to have an even number and to never, never have 3. No matter how good of friends they are one is always left out at some point.
I also believe that sometimes it's easier to have a friend over. The two girls play together which give me more time to do the things I want to do.
There is no right way to help your child spend time with friends. It is important that they have opportunities to do so, especially in the summer. Do what is the most comfortable for you. You will feel anxious until you've developed a routine that works for you. Start out with a play date so that you have the experience of managing another child or children in your home. Then have a sleep over.
When I first started having play dates and sleep overs for my daughter I had a difficult time because I didn't set out the rules and expectations out front. But I caught on to how to do that by watching other mothers when I took my daughter to their home.
I don't mind if the kids ask at the last minute or when the friend is with my granddaughter. I'm a spur of the moment person anyway. My daughter wants to be asked ahead of time and without other kids present. You do what you want to do. If one way doesn't work, try a different way.
I also worry sometimes about my granddaughter over staying her welcome. In reality nearly all mothers will directly say what time to pick her up or will call when the kids are not getting along so that I can pick her up. We adults are in the same boat and just seem to naturally work together. We are arranging activities for our kids and not needing to impress each other. The kids try to do that for themselves, Impress each other. lol We parents laugh together as we take care of ourselves too.