For competitive play I never go with out at least a unit of 6.
Because of their scouting your opponent have to take them in consideration when lining up his forces (and indeed his war-machines) in deployment, and it might cause him a lot of irritation for close to no points.
short answer: yes, they are very good at hunting warmachines.
longer answer:
I prefer them over Terradons. I have not yet tried rippers for warmachine hunting so I cant say anything about that for sure, but I know they wouldn't work (for warmachines) in my meta, as too much is between them and those machines for them to statistically NOT charge anything else due to frenzy.
when answering to "how many" the answer will be "how many can you fit in?" in both terms of what your playstyle is and if there is actual space for them to slip through you opponents ranks to get to the juicy bits behind their lines.
3 or 4 wide is what works best for me, meaning units of either 6 or 8.
When it comes to skirmishers like chameleons I like more small units rather than a few big ones.
This also makes it even harder for your opponents deployment.
Where they DO fall short compared to terradons and rippers is that they dont fly.
My friends have gotten used to my Chameleons so they know how to place themselves (yet I still bring em, as its still to my advantage). This means my chameleons cant get through, and usually that means that they end up assisting other skirmisher units in making needle-cousins of other targets.
I am starting to think I should bring both chameleons and some flyers...possibly rippers, I dont know yet.
Try two units of 6 to start out with and see if they can get the job done

hope it helps!