I've got 20,000 trees right now, mostly started from 1 year old pruchased plugs, although I start a lot of my own poplar and willows from cuttings.
You can't grow everything you sell, unless you sell a limited market. I grow most of what I sell, but I track requests, and will buy trees from other nurseries, 5 or 10 each for anything I get more than one request for in a year. On the average half of those go in a year, and the other half move to a larger pot for next year.
I don't figure it's worth bothering with less than 300 of anything. Each batch requires individual attention in the watering cycle, althoguh I try to group similar things together.
If you are starting lots of things, look at styroblocks. They outlast the cheap plastic cells -- good for 10 years if you store them out of the sun when they aren't working. Each block is 13" by 23" and has 15 to 120 cells in it, range in volume from 40 ml to 1000 ml. Overwintering is reasonable with these.
Another trick: For seeds, just start them in large trays, then transplant as soon as they ahve 2-3 true leaves into #1 pots.
I have two streams of transplant: Styroblock -> #2 pot -> #7 pail and #1 pot -> #5 pot -> #10 growbag. In each case the size differential permits good access to pack around the edges.