The pouches on this page are all thickness 2x 42 micron and are sold in boxes of 100 sheets.
The larger sizes like A4 and A3 can be purchased in larger quantities for better prices.
If you are unsure what thickness of pouch you need, here is recommendation:
The thinner the paper the thicker your pouch should be. For example: Paper of 80 gr/m² should best be laminated with 2x 125 microns or even 2x 175 microns. 2x 80 microns will not give this document the strenght it requires.
Thick cardboard can best be laminated with a thinner pouch like 2x 80 microns or even 2x 42 microns.