It's not the number of roots that matters, it's the overall size of the vine and whether you can irrigate it during dry spells. I have 2 concord grape vines, have never pruned them, they are about 20 feet long and produce 30+ bunches of perfectly edible grapes each. Although I do wonder if there's anything I could do to make them produce less acidic grapes, I have trouble getting them eaten because they give everyone heartburn. :P

Reliance is supposed to produce best with 40 clusters.
I don't think Himrod is prone to over-production and you kind of be happy with what you get.

Search and see if you can find a University study on the varieties you have.

Bunchgrapes has some pretty good variety specific information.

Here is a link that might be useful: Bunch Grape variety list

Sautesmom, What kind of history do you have with those grapes and what kind are they? As long as those vines are capable of producing full-sized grapes, I wouldn't worry about pruning excessively. In fact, I consider 30-50 average. It's hardly worth having the vines for only 5 bunches.

S and S, Concord is not a table grape; and altho' I enjoy them out of hand, they are best used processed into grape juice and jelly.

Brook

I have one variety that makes more clusters AFTER I think I thinned them. It's a wine grape. Tempranillo. It amazes me that new clusters form when you have thinned. It's biology, I guess. The vine wants to reproduce itself. And grapes are it's seeds.

I wouldn't prune them this year. But when they go dormant, I surely would. I'd be more worried about the squirrels and possums!

Good luck, and I hope they save some for YOU!
Suzi

New clusters can form around and after thinning but they are not normally on last years wood like the early clusters are. They form when this years buds are forced to grow. Normally this years buds don't grow this year. They wait and grow next year and give us next years crop of fruit. But if this years shoots are cut back part way, this years buds lower on the shoot will grow and those shoots often set a cluster of grapes.

Don't know of any other fruit that sets fruit buds so fast on new wood. And those fruit buds need no rest or cold to set fruit.

brook - Concord is sold at the grocery stores around here as a table grape, but yeah I know it's not officially one. The seeds are a pain and the skins difficult to digest. Other than that it would make a good table grape.

I was wondering if making the soil less acid would make the grapes less acid, but when I googled grape acidity people were saying it's mainly the weather when they are getting ripe, too much rain makes them less sweet.

For maximum sugar content, grapes must have maximum sunlight exposure. This means no shade. You may also try whats called shoot positioning and leaf pulling to get the available sun's rays to strike the clusters. Paul