Only one pair of boots affects your movespeed.. The highest bonus boots is the one taken.. So if you buy boots of swiftness and boots of mobility, in combat you'll only have +3.. Out of combat you'll have +5. But don't buy two pair of boots.. Its bad.

If you want to see some crazy speed, get a rammus to stack boots of mobility with 5 phantom dancers. Have him power ball and pop ghost
then have Zilean cast haste on him.

Rofl.

And yes, only one pair of boots per inventory. Everything else is a waste of space.

Only items that say +X% move speed stack. Enhanced Movement items (boots) will only increase your speed by the greatest level you have.