Everything bike related I've owned took 3, 4, 4.5, 5, 6, or 8mm. The 6 and 8 require more torque than you'll get with a T-handle. The 8mm fasteners need more than a hex key or standard length socket wrench (especially for removal).

P-handle ball-end are the best option - you get the torque of a conventional L-key when you needed it, can spin fasteners in like a T-handle, and the ball tolerates misalignment.

You may also need a few Torx keys - Campagnolo uses T25 on their shifters (T or P handle optimal for clearance), and T27 is common on alloy chainring bolts/nuts.