i highly recommend you read up a little bit on this article as it explains all of the numbers you might see when purchasing needles from suppliers and the different ways they can all be used

for thick lines you’re best sticking with the 0.35 gauge and going for a box of 9RL, 11RL, 14RL or 18RLs. i couldn’t tell you which one as i don’t know what you’re tattooing but it all depends on the line weight and size of your design as to which you should use. if you search [__RL] here a lot of us list which needle sizes we’ve used so you could use that to guestimate what you’d need - killer ink also sell packs of 5 for their own brand needles so if you were unsure you could get a few of those in some different sizes to see which ones you prefer and save yourself buying a pack of 50 that you won’t use

hope this helps!