Re: certificate of employment, is it a company you work for, not one you own? Reason I ask is I’m assuming that the expectations will be different in each case.
You enter as many times as you like - and each time you enter, you get stamped in for 180 days, which you’re allowed to extend a further 180 days at immigration. “Once” means you can’t keep extending at immigration, you can only do it once, then leave and come back.
jeeze mate, I was basically suggesting to just chill out, but seems you’ve got quite a chip on your shoulder. I’ll spell it out more explicitly… re-read my comment, it was only intended as a bit of banter/fun. It’s Friday arvo, you’re in Thailand, enjoy a beer. Cheers 🍻