Honey, thread gets all the glory, but the needle is the one doing the stabbing. Note 016 covered thread weight and the rule about going up a needle size when you go up a thread size. This one is about what kind of needle, and why waxed canvas is so unforgiving about it.
Waxed canvas is dense plain-weave cotton with wax worked into the fibres. Ordinary cloth closes up around a needle hole. Wax doesn't. Every hole is permanent, a tiny leak and a visible mark. The needle has to pierce the weave cleanly, because you only get one go at it. There's more on the cloth itself in the waxed canvas reference.
Use a sharp point, not a ballpoint. Ballpoints are for knits, where you want to slide between loops. This is woven cloth, so you want to part the threads cleanly. For 10–12oz canvas I start around 90/14 and go up to 100/16. I keep 110/18 for stacked layers like strap ends and handle joins, where the extra layers need the muscle. Denim or jeans needles are a good default. Leather needles have a cutting point, and on waxed canvas that means tearing and perforating. Over a long stitch line you end up with something close to a postage stamp. Skip them.
Wax builds up on the needle and turns it gummy, which drags the thread and skips stitches. Wipe it down often, or swap it. Swap early, too. A dull needle punches instead of piercing, and a punched hole is bigger and uglier. A fresh needle costs very little next to a ruined panel.
Before you commit, test on an offcut folded to the real layer count. That's where you find out whether the size is right, especially at a bound seam where the layers pile up.
And remember, you can't unpick waxed canvas cleanly. The wax shows every needle mark, like a plot hole that never goes away. If you do get stray holes, rewarm the area gently with a hairdryer and the wax will move to close them up. It helps, but it won't save you from a bad first stitch. Measure twice, stab once.