The general rule of thumb is one MH per 2 feet. Of course the exact light spread depends on reflectors and positioning. Three would probably do the job well. Two should be fine, I doubt any dimmer areas towards the edges of the focus area will be too obvious.
Also, you can get a bluish color with T5s if that's what you want. Get actinic tubes or higher color temp ones. Just like MHs, it's all about bulb selection.