"10" means that there should be ten perforations within a two-centimetre length. If a stamp is decribed as "perf 10", it means that each side has that perforation. Where a combined perforation is shown, eg 10 x 12, it means that the horizontal edges are 10 and the vertical edges, 12. You need to get a perforation gauge to check perforations, not count holes!
This is probably the most widely used one in the UK. Lighthouse also makes a more basic modelhttps://www.ebay.co.uk/itm/Stanley-...AOSwjthcNnWZ