I have installed a back splash in my kitchen, if I use the plastic spacers how many should I use or is there a limit? I understand that this can cause to much air space around the receptacle or light switch.
If your problem is that the thickness of the back splash material has caused the electrical boxes to become recessed you may need to use electrical box extenders. These come in a number of styles depending on the type of electrical boxes you have. There are options for metal or plastic boxes for starters. Other factors would be amount of extension needed and whether you are dealing with a single width type box or a wider type.
Here is an example of a metal box extender that works in a telescopic manner to accommodate just the amount of extension that you require.
If you're in an area that follows National Electrical Code (NEC), the boxes cannot be set back more than 1/4" (6 mm). If the backsplash has caused the boxes to be set back more than this, you'll have to use extenders to bring the box out to the proper depth.
If the box is less than 1/4" from the finished surface, you can use as many spacers as needed to mount the receptacle in such a way that the yoke is held rigidly at the finished surface.
You use as many spacers as needed to get the outlet/switch mounting tabs flush with the tile. You may not need them if the mounting tabs of the box extender suggested by Mr Karas are tall enough to hold extender and device (just need longer screws: 6-32, in either case )
Die cut wallplate insulation gaskets: they go over switch/outlet and under wallplate. They block airflow by sealing around box perimeter and fit tight around device.
The adhesive is good for 1 shot and then will need replacement.