My earlier answer counted 4 (I don't recall the exact source now), but I saw "2" in some other answer.
What's the correct number?
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Just re-watched the DVD. Cut off 2 front paws, then did a uber-cool lightsaber twirl and stuck the lightsaber into its cranium.
My copy of the adult novelization only mentions that he CUT his limb, but not cut any OFF:
“Straight ahead,” Obi-Wan told himself as the acklay came at him, its huge claws snapping in the air.
He went left, then right, then rolled forward at the beast, between the mighty arms and snapping claws, coming around and over with his borrowed lightsaber stabbing straight ahead, burning a hole in the creature’s chest.
The acklay dived forward, trying to crush him under its bulk, and the Jedi leapt straight up as he connected. He came down on its back, landing lightly and stabbing repeatedly, before leaping away once more.
“Straight ahead,” he told himself again as the enraged beast charged yet again.
Obi-Wan noted the blaster bolt coming at him from the side at the very last second, and turned his lightsaber down and under, deflecting the bolt right into the acklay’s face.
The creature hardly slowed and the Jedi had to throw himself to the ground to dodge a swiping, snapping claw.
He rolled out to the side, to avoid a stomping leg, and managed to slash out again, cutting a deep gash.
The acklay howled and came on, and more blaster bolts came at the Jedi.
His lightsaber worked furiously, brilliantly, turning one bolt after another right into the charging beast, finally slowing it and stunning it.
Obi-Wan rushed in and leapt and stabbed, right in the face. He caught his foot on the creature’s shoulder and ran right past it. He heard it fall behind him, thrashing in its death throes, but he knew that battle was done and went back to work on the battle droids.
Patricia Werde's Junior novelization doens't even mention that fight:
There are too many of them! Obi-Wan lost count of the droids he had destroyed. He could feel Jedi dying around him, overwhelmed by sheer numbers. The sand of the arena was soaked with blood and littered with droid parts, and more Battle Droids were still coming. The three execution-monsters were dead—Obi-Wan vaguely remembered killing the acklay himself—but that hardly seemed to matter.