I currently have 17 up and playing at the house, but a few of those are probably going to go so I have more elbow room to do some other things. I've owned well over 100 machines at once (buying projects/warehouses etc) and have bought, fixed up, played and sold/traded many more. I only have a total of about 25 now and no storage that I'm paying for, but a few are at another location getting ready to be sold off.
I plan to keep the total number smaller in the future, but will most likely keep switching them out as I feel like it.