It starts a 2, +1 if you have a 1 in int, +2 if you have a 3 in it. Beyond that it never improves unless you have a power that specifically says so. Also, at 5 (and every 2 levels above 5, so 5, 7, 9, etcetera), you get an extra Mild consequence of the appropriate type.
So say you have a 5 in endurance, you would have a physical stress track of P OOOO, and you would also have +1 Mild Physical Consequence.