I think the first suggestion you're likely to get here is to stay away from peat pots. They're supposed to break down in the soil, but very often don't, and you end up with an incredibly root-bound plant before it dies. If you do use peat pots, then strip most of the pot off before planting out. At least get rid of the bottom. Lots of dislike about peat pots around here.

As to container size, I know people who seed in those tiny 6-pack plastic containers you get in the nursery, and then plant into larger containers when the first leaves come out. I plant directly into 4-inch plastic pots, and keep them there until they are ready to plant out - maybe 4-5 inches tall in the case of tomatoes. Shuffling seedlings between containers just seems to me like an unnecessary step. But I'm open-minded. Maybe someone can provide some justification for doing it.

Of course, one argument for planting in small pots is that you can fit more of them under lights. But you'll eventually need those lights for the bigger pots.

In those 4-inch pots, I thin to three plants, and then to two. I plant out with those two plants per hole. The combined production of those two plants will equal that of one, but I consider the second plant as insurance against a hole with nothing in it.

One 4 inch pot with onion seeds? Well, I guess it depends a lot on how many onions you want! I don't start onions in pots.

As Dan said the common recommendation is to avoid using the peat pots...period. But if you must then don't just cut off the bottom when planting - strip off the entire pot first. You will also have problems growing seeds in them because they screw with soil moisture by wicking it out of the soil leading the grower to over-water the seedlings. You'll do much better if you toss them and use just about any plastic container you can find with holes punched in the bottom.

As to number of seeds per container - that's a personal choice. It is very late to be starting onions from seed - try January next time - but one can easily grow 20-25+ onion plants in a 4" pot. No way would you plant just one.

For the other vegetables each person has their own opinion as to how many in each pot. I only use 4" pots for transplants and never for seedlings and I start large numbers of seeds in each container and then separate them once sprouted into individual containers. In part it depends on how many lights you have. You can't fit much under 1 or 2 lights.

There are several other growing from seed discussions running on this forum right now, not to mention over on the Growing from Seed forum here. If you will read through some of them you'll find most of the basics already being discussed.

Dave

This is what I'm using. Each cell is 2 1/2'' W x 2 1/4" L and 3" deep https://www.amazon.com/starter-trays-EXTRA-LARGE-labels/dp/B00HTCUBZ6/ref=sr_1_1?ie=UTF8&qid=1489001691&sr=8-1&keywords=seed+starter+trays+90+deep+extra+large+cells