Ten years ago, pretty much by accident, I became an instructor with the grand total of 140 logged dives, all done in benign tropical conditions. 4000-odd dives later, from my lofty pillar of dive-godliness, I look back and am amazed they let me in the water, let alone teach people to dive.
Here's the thing, though. I think I'm probably an ok instructor - not Earth-shatteringly great, but competent, good on the technical stuff - and a very competent diver. Ten years ago, in all honesty, while I wasn't anywhere near as strong a diver, I was probably a better instructor than I am now, certainly at OW level. I had an enthusiasm for teaching that might, er, be less obvious now...
Entry-level diving in tropical conditions (which is what the vast majority of new divers want) isn't rocket surgery, and the agencies have good enough programmes in place that a conscientious instructor with an enthusiasm for teaching can do a good job. You need enough experience to know whether a student's doing things properly and you need to be comfortable enough in the water that you can deal with the minor emergencies of an Open Water course without being too stretched in terms of your own dive skills. And that's about it.
After ten years in SE Asia and the South Pacific I wouldn't presume to teach anyone wreck diving in the North Atlantic without building up some solid experience in those conditions, but there's a breed of instructor out there who does that. For most other folks, the zero-to-hero instructor - as long as they have the teaching ability - almost certainly has sufficient skill and experience to get them started on diving.
Whether the world needs another 100,000 SCUBA instructors (or whatever the number is) being produced annually, now, that's a whole other question...