You only need one, I've never had any issues with just having one and have been airbrushing with the same compressor since '94.
Essential bit of kit. Mine doubles as my pressure control too.
Important stuff to do with your compressor though. When you finish ( or even before you start ) press the button on the bottom of the moisture trap to get out any moisture. Also every time you're done, empty the compressor tank, this is important !
I also disconnect my airbrush hose and leave it hung over something, that way if there is anything in it then it can drain out over night.
Like I said, never had any problems.
I used someone's airbrush once with a moisture trap on the bottom of the airbrush, just made it cumbersome and not so well balanced.
I'd buy one, spend as much as you can on it, this is ideal but I know everything airbrush do a similar one for a third of this price
https://airbrushes.com/product_info....d5edeb83a5735a Once you've bought it and fitted it, see if you get any moisture through, my guess is you won't, but if you do then think about investing in a 2nd one.