Two ways this could go:
1) Trainer could rent stalls to boarders, taking on all responsibilities, feeding cleaning, etc. Most trainers would then give boarders lessons and or train/ride said boarders horses several times a week. They would then collect monthly board from the boarders. Around here my friend pays her trainer $700 a month for this service.
Your place would essentially be a boarding barn with people coming and going, but the trainer would be running it. Boarders also might want to keep their horse trailers at the barn, a lot of stables charge extra for that. So if you don't want horse trailers you need to discuss that with the trainer. My employer hated to look at the trailer, I couldn't keep it where I worked.
2) the trainer could take horses in for training with out the owners. You would have the owners coming out to see their horses, but it would most likely be by appointment with the trainer.
This senario might be more difficult to fill, because it might be hard for one trainer to keep 6 stalls filled all the time.
You can set up ground rules with the trainer, like when the barn closes for the evening, and the maximum number of horses on the property.
This doesn't answer you how much question.
You need to figure out your expenses, electric, maintenance, etc.
But you need also to realize that the trainer need to make a living also.
It would be great if you got an awesome trainer, that was a proud of your barn as you are and takes care of it accordingly.
Don't charge per horse because that gives the trainer no incentive to keep the barn full, why is it your loss that she lost a client.
I will ask how much my friends trainer pays for her barn.