Some would say 6 bulb, some would say 8, some would say as many as you could possibly fit. If I were you, I would leave a little space front to back to add reefbrite strips to complement the t5 bulbs. ATI is a well known and trusted fixture if you want to hang one, and reefbrite strips are made with brackets to hang from the screws already designed into the ATI fixture's exterior. You could also find some cool german stff from time to time...or go DIY rigging.
ATI bulbs are the most used whatever fixture you used. Ask folks running the same fixture as you select what they like for bulb selections so you can get a feel for color rendering. On a four-bulb I have reefbrite tech led strip, P+, B+, C+, B+ and have always liked it. I like it best when there is one reefbrite tech and the C+,B+ back bulbs on.