The answer to your question depends mostly on the type or style of pizza that you want to make. If you look at the matter of pizza dough hydration from 30,000 feet, the range of hydration values you will see can run from the mid-thirties percent to close to 100%. If you examine the individual styles, the answer becomes much clearer and easier to understand.
For example, a NY style dough formulation typically has a hydration value of from about 58% to about 65% (see http://pmq.com/tt2/recipe/view/id_151/title_New-York-Style-Pizza/
). However, as a practical matter, in a commercial setting, a hydration of around 58% is quite common.
An American style dough formulation, such as represented by a Papa John's or Domino's type of dough, can have a hydration of around 56-60%. However, American style doughs usually contain a fair amount of oil that also has a "wetting" effect on the dough. So, the hydration due to water content can't be so high as to make the dough hard to handle due to extensive extensibility (stretchiness). American style doughs tend not to exceed about 7% oil so as a practical matter that tends to limit the hydration to around 56-58%. For an example of the interplay between hydration and oil for an American style dough, see Reply 2 at http://www.pizzamaking.com/forum/index.php/topic,6758.msg58197.html#msg58197.
A Chicago deep-dish dough has a similar interplay between hydration and oil but the oil can typically range from about 8% to well over 20%. The value of oil used will typically dictate the formula hydration. You can see an example of the hydration-oil interplay for the Chicago deep-dish style at http://www.pizzamaking.com/forum/index.php/topic,6480.msg55567.html#msg55567.
A Neapolitan dough formulation, especially one using 00 flour, can have a hydration value of around 55-60%. However, there are some professionals, and some of our members as well, who have used hydrations in excess of 65%. This latter value is not something for amateurs. It is hard to do and takes a lot of skill. And it is pretty much limited to applications where a very high temperature oven, such as a wood-fired oven, is used. For a home setting where a standard home oven (electric or gas) is used, the hydration can't realistically be more than about 58-60%.
A Sicilian style dough formulation can have a hydration that can range from the mid-sixties percent to over 80%. This pretty much means that Sicilian style pizzas have to be baked in pans.
A cracker-style dough formulation typically has the lowest hydration of all pizza styles, with a hydration in the mid-thirties percent being quite common. For an example of such a dough formulation, see Reply 16 at http://www.pizzamaking.com/forum/index.php/topic,5762.msg49138.html#msg49138.
There are also many examples of no-knead and similar high hydration dough where the hydration can range from around 75% to close to 100%. An example of the latter is the rustic ciabatta pizza dough formulation at http://www.pizzamaking.com/forum/index.php/topic,8539.msg73858.html#msg73858.
It takes a lot of skill to master working with doughs with such high hydration values.
The above is not intended to be an exhaustive analysis of the hydration matter. There are many dough formulations that fall in the cracks between the above examples and we have many members who have found ways of using higher hydrations than the values mentioned above, in many cases using autolysis and stretch and folds and the like to be able to come up with doughs that can be handled without much difficulty despite the high hydration values. But I think my analysis gives you an idea as to the wide variations in hydration values that different types and styles of doughs can have.
EDIT (3/22/13): For the updated link to the PMQ recipe, see http://www.pmq.com/Recipe-Bank/index.php/name/New-York-Style-Pizza/record/57724/