I generally don't expect skin to be completely clean of a particular scent until I've had a bath/shower (and scents can go on evolving and changing for hours and hours anyway).

So if I want to test multiple things at once, I'll use different patches of skin on my hands/wrists/forearms, trying to keep the patches for different scents as distant from each other as possible.

Generally I'll only test two or three new scents on skin at the same time; too many and my sense of smell gets confused/fatigued.

The exception is when I already know the individual scents and I just want to remind myself of what they're like -- e.g. when I'm writing up a perfume post. *g*