That depends on a lot of things.

What kind and size containers are you planning to use using?

What size are the containers of potting mix?

What brand of potting mix are you talking about?

Good to plan ahead though, and you already have some of this figured out. If you plan on doing 100, expect to add a minimum of 50 more and that is really a bare minimum.

You will get more answer and more exact answers when you can answer the questions I asked.

Fran

I winter sowed 300 containers this year and used 2 compressed bales of Fafard growers mix. They're 3.2 liter bales. I used gallon milk jugs and filled the bottoms with enough growers mix so that it was 4" deep after water was added. Winter sown plants have incredible roots and they use that soil/mix. Here's what my container ghetto looks like:

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Everything in these pictures was winter sown this year except the lambs ear in photo #1 and the variegated iris + columbine in photo #3. I buy the bales of growers mix in August from a nearby nursery supply and stack them inside my garage so they aren't frozen when winter sowing season rolls around.

Fran - I don't know what the containers will be - they will be a variety, though no milk jugs since we are a lactose free house. So far I have a few seltzer bottles and some juice jugs. If I don't get enough containers thru normal household consumption, I may end up buying empty milk jugs.

I don't know what size the bags of soil will be, or the brand. The notice from the hardware said that all potting soil, top soil, manure etc. is on sale. Personally I find smaller bags more manageable, but if larger bags are cheaper, I may go with larger bags.

gardenweed's answer was perfect - if I decide to go with Fafards growers mix, I know to buy 1 compressed bale.