I do germinate tomatoes in a flat container. I use mushroom plastic containers which I put holes in the bottom. When the tomatoes have their second true leaves I transplant them into 16 oz glasses sinking them up to the leaves. Tomatoes--no other plant--will grow roots all along the stem. Mine stay in these until they either get too big for the pot or are planted out. This would usually depend on when they were planted but most times they stay in the 16 oz. glasses. I don't like transplanting too much because every time you do you set the plant bach a couple of weeks. I do the same with peppers but I don't sink them down.

I gave up planting anything in the cabbage family because I got tired of fighting the cabbage worm

There is no set formula or time or number of leaves. It is done "as needed". Many things would never need to be transplanted, especially shallow rooted vegetables.

This is all assuming they are started at the right time rather than way too far in advance. Anything that is started inside too far in advance of the date it can go to the garden outside may end up needing multiple transplants and can easily end up too large to tolerate transplanting.

Dave

You don't have to transplant anything to bigger pots. Don't make it harder than it has to be. Research the last frost date in your area (which is in May). Look on the packet or online for how many weeks before last frost to start seeds indoors, then plant them at that time. You can harden them outdoors and plant directly in the garden.

Take notes to see which plants outgrow the containers and use bigger containers next year. Tomatoes, castor beans, petunias use a larger starting container (4") to start. In my experience, broccoli can be direct seeded.

Transplant to larger containers when roots start to fill container and/or have to be constantly watered. Leaf size is irrelevant.

Potting up is one of my least favorite things to do but it is sometimes the best thing to do in my opinion. If you start in too large of a container then seedlings may grow slowly and have a tendency towards root rot. If you leave them too long in small containers then keeping them in those containers can lead to a challenge keeping them from drying out. Tapping them out of their containers and looking to see how root bound they are getting can be a decent baseline for potting up.

Yep...............you are going to quickly realize that there are many different ways to go about getting your garden started. I am the one who suggested the potting up in the different size plastic cups - it works well for me, but this time of year I like to fuss with my seedlings. My experience (little though it may be) is that leaving them in containers that are to small for them is much worse than the slight trauma of re-potting. I tend to start things to early, and need to move them to larger containers, but I have a short growing season. I am harvesting pretty much everything a good two weeks before my neighbor who buys his transplants in 6 packs or direct seeds.