hello , i have a multi voltage adapter that can range from 3V
up to 12v . i want to run multiple pedals from it .

it says it has a 500mA maximum how many pedals
on a 9V setting should i be able to run ?

or should i not use a regular adapter at all ?

thx in advance
For one thing, those are not regulated.and have very minimal filtering. Aside from humming more than a regulated supply, the rated voltage output is only valid at the rated load. So that thing will out out 9v at a 500 mA load but could put out as much as 15v with no load.

Personally I prefer to use regulated supplies for pedals but you could use a generic supply if you had to.
thx for the response , is the other adapters they sell like the " DC on tap - rocktron "
a regulated power supply ? which is just an adapter like this one .

there are several of these adapter supply's that i'm sure you have seen online .
The rocktron is a switching supply so it is regulated. Power supplies intended for pedals are generally regulated in some way. Due to our wonderful modern technology, switching supplies are much smaller, lighter, more efficient, and cheaper to make than old "brute-force" supplies that just use a transformer and a couple of diodes and a filter cap.
if " wall warts " are regulated what is the difference in using an expensive
VOODO LABS psu over the ROCKTRON psu ?
Brand name and build quality, mostly. Some may have extra features like some form of current limiting or short-circuit protection, switchable voltage selection, or "dying battery" simulation.