How many plants fit in an 80x80 cm grow tent?

0.64 m² is not much floor. Most people do better with one or two trained plants that fill the space evenly than with four that fight for light and air. Keep a gap around the pots so you can reach every side and fit a clip fan.

For light, a real draw of roughly 150-250 W is about right at this size. Have a look at the 80x80 complete kits, or buy the tent on its own and add an LED grow light that fits.