I started with 20; put them in, and then decided I had space for 50 and made another order and got them. Now going into my second year . If you can do 20 I think you can do 50 labor wise and economy of scale, if the site is convenient and you can go out and putter around anytime you please. I have other things going on besides the vines so as one poster said above, you can go from a hobby to having a second job pretty quick.
In my situation, at the end of the day its "me, myself and I" as the caretaker of it all. I have volunteers for future harvests, but who knows if they can make it when needed. And my kids may not be interested either. If the deer get a few of my grapes Im not going to care too much.
As a one-man operation, 50 - 75 would be my limit, and again that depends on location and harvest goals. I find 50 to be pretty good balance but I think I could do 75 and not burn out or get overwhelmed. 100 would be too much for me, in my situation.