I've always had success using Dixondale. They offer a nice onion called Texas Super Sweet that does great in my area. I haven't come upon another that grows so well in my particular area and climate. So why not stick with success.
Last year I ordered 10 bunches, which supposedly should be 50 plants per bunch times 10 bunches = 500 onions. In reality, I end up with a lot more, almost twice as much, because the bunches are generous and contain lots of small seedlings which I nurse along in the mini greenhouses. Each bunch has about 50 hearty plants that go right out into the gardens, plus 40-50 little seedlings that I'll let grow up for a few weeks in containers. When they are big enough, they'll join the others in the garden beds.
Last year I didn't have enough onions and ended up growing lots of green onions from seed. Not enough onions, how could that be? Well, Adam and Matt also love onions, so between them and us, we go through a lot. On top of that, fresh onions are easy to sell and trade here.
This year I'm ordering twice as many onions. I just ordered my usual 10 bunches. In another week or two I'll order another 10. The reason for the delay is that I need to get the growing beds and mini greenhouses ready for them. I'm not prepared to handle all of them at once. So two shipments is doable. I'll also order 10 bunches of leeks once I get all the onions planted. Then as insurance, and because we like green onions, I'll sow some onion seed too.
By the way, I think I've already mentioned about how to double harvest leeks by cutting the stalk off at ground level and allowing the base to regrow. It works with green onions too, though they are more apt to bolt to flowering. This way I can dramatically increase my leeks and green onions without having to be growing extra beds of plants. I've learned that this method only works well in fertile soil.