I have a fixture with a 660 in it and have done some reasearch on this subject.
In order of your questions.
How many - 4ea (no it will not run more)
Yes they overdrive a traditional 58w (+/- cant remember) bulb to about 80 watts.
The lamps will last up to 18 months..... IF they are actively cooled. There was a test done by a RC member called GrimReefer. When T5's are not cooled (by a fan blowing across them) and the temp of the bulbs exceeds 94 degrees (+/-) the output (PAR) of the lamp drops off dramatically. If I recall correctly he also suggested that the excess heat would probably shorten the life of the lamp. Even if he didnt suggest that, I would think it to be a likely scenario.
If you are building a hood, it's a good idea to put fans in the hood. The canopy I purchased is by aquactinics and it has IceCap variable speed fans built into it. They are pretty quiet and work well.
You can also ask other questions directly to IceCap and Aquactinics in the vendors forum. Both companies have forums at the bottom of the main forum page.
Hope this helps