Well, he had the time to track them down and kill them, I don't think it was any issue to him if he had to kill 10 Aliens or a 100. And when you look at the aliens, with the mindset that they were retarded because what they didn't do when they had the oppertunity, lost their acid effect and seemed overall easier to kill, then you have an situation where a seasoned Predator tasks himself with the misson of hunting down a bunch of aliens with Down's syndrome.
I totally agree that those Aliens were retarded. But I don't see Wolf killing hundreds or thousands of them without his guns and other weapons which he had lost near the end of the film. He even lost the blue liquid which was his only way to cover up the whole thing. He's supposed to be a "cleaner" after all. All in all Wolf's just as retarded as the Aliens.
At the rooftop he had his spear, whip and wristblades, if he would have won the battle with the predalien then he could have picked up the shuriken he left at the hospital, so he would have long, middle and close ranged weapons as he dealt with the rest of the aliens.
I think his main mission was to find the crashed ship and get rid of it. It was intended that he found out about the Predalien and the escaped facehuggers as he found the ship on earth. There his mission changed from a rutine clean up mission, now it's all about damage control.
As a cleaner I think he knew more ways to get rid of a body than useing the blue liquid but I think that getting rid of bodies was a secondary objective, tracking and killing the Predalien was his main objective after the crashed ship was gone. But I still can't figure out why he approached some situations the way he did, like in the sewers when he used himself as bait. I would have liked him to use some sort of gadget to lure them to it, something that used a scent or sound and then get the jump on the Aliens by useing the element of surprise. You don't see a hunter put himself in a corner then cover himself in sebra piss, with the intention to let a pack of lions have a go at him, thats just stupid but it's more or less what Wolf did, he didn't use piss, instead he used his roar to get their attention but that didn't take away the stupidity from it.
I don't think that there was a hundred or thousands of Aliens around, the Aliens who didn't run with the Predalien where killing people that could have been used as potential hosts and the Predalien didn't use everyone it encountered as a host, some it used as hosts and some it killed on sight. So by simple mathematics there couldn't have been hundred or thousands of them around.