Each battle tank that expends its full movement allowance to clear wreckage removes one wreck.
This is cleared up and clarified in the published errata and Q/A for AIW, pasted below:
Q: How are wrecks removed? A: A battle tank unit that starts the turn adjacent to the wreck counter expends its whole movement allowance to move one hex into the hex with the wreck. As the tank enters the hex, the player removes one stacking point’s worth of wrecks. Two tanks can do this to remove two points worth of wrecks.