Here's what I do. I am sure that there are many other approaches to this.
I start most everything in cookie boxes - you know, those clear plastic boxes you get cookies in at the bakery. The smaller ones for muffins, single-serving cake and sandwiches should be suitable for most gardeners. I've found that the bakery people are usually happy to sell these empty but I try to help them out by buying them full
. Just keep collecting them in a corner of the garage.
When the 2nd set of leaves (1st true leaves) show up on the seedling, it is usually about time for a larger container. Most things go in a 48-cell insert and a tray. Snapdragons and some smaller, slower-growing plants (like the perennial herbs) can go in 72-cell inserts.
Yes, I often transplant sunflowers. They start off in the 72-cell inserts, 1 seed in each. Cucurbits (squash, melons, cucumbers) go 1 seed each in 48-cell inserts. The idea is that I'm setting these plants out in the garden after just a few weeks. They grow quickly and do not transplant easily.
Tomatoes will be moved from the communal
cookie box into 48-cell inserts and then, usually, into 4" square pots that can go 18/tray. Those pots are good until the plants are 8 to 10 weeks old altho' the final couple of weeks it would be best if they aren't packed
so tight as 18/tray.
Hope that gives you some ideas.
Make everything as simple as possible but not simpler. ~ Albert Einstein