Lower number equates to smaller tip size. 00 very small and goes up from there. The brand of torch handle should have a tip chart on line. Tip chart will tell size of material it can cut efficiently and recommended pressure. Pressures vary a little with tip size as well. FOR ME 30psi oxygen and 5 psi acetylene for smaller tips and small material work well. I will go 40 and 7 on heavier material but use more gas.. Reference the tip chart to be sure for your setup!
Your welding shop should have a brand specific plastized card with torch settings and thicknesses. Then you can have it with the torch so you will actually use it. Same goes with welding calculators for mig welders.
Should be a dollar or two. If you use the chart and don't over tip you can get slag free and very smooth cuts with a torch. Well that is if you have a smooth hand as well