Are you intending to use liquid yeast or continue to use dry yeast?
Starters are most useful for liquid yeasts, but you could make them from dry yeast in a similar way. The kicker is, dry yeasts (e.g., Safale US-05 or S-04) already have a fairly high cell count of around 200 billion viable cells per pouch. You need a fairly large starter volume for them to propagate.
There's a lot of information on making yeast starters already, such as the stickies on the top of this forum. A search will yield a lot of useful information. Most are for 5-10 gallon batches, but the principles are the same for larger, 20 gallon ones.
When you've done your research, ask specific questions of things that aren't clear to you or what's needed in your specific case.
I'm going to give you an overview of what's involved in making starters. There are other approaches:
Have you looked at a yeast calculator and put some typical numbers into it? That may help giving you a better idea of the various parameters involved. I usually use this one at
BrewUnited's. There are others.
Do I understand correctly you are going to brew 20 gallon batches?
If so, BrewUnited will tell you that for 20 gallons of 1.060 ale wort you'll need approx. 837 billion cells. That would equate to 4 pouches of US-05, for example. That's the easiest, and if bought in volume or 500g bulk packs, perhaps the most economical.
Yeast can be reused/repitched. For example, you could brew a small, 5 gallon medium gravity batch and use the yeast cake from that to pitch into a 20 gallon batch. That gets you beer and yeast. Your 20 gallon batch then generates enough yeast for 4 more 20 gallon batches and so on. Most breweries repitch yeast, 'cone to cone.'
Liquid yeast typically comes in 100-200 billion cell packages for homebrewers, depending on the yeast lab. Larger, ready to pitch packages are available in different sizes for craft breweries. You're sort of in between, so you may want to stay at homebrew level there.
The cell count is stated
at the time of packaging at the yeast manufacturer. The viability of the yeast goes down from there, cells die, even if stored refrigerated at 34-36F. So a yeast pouch that is 3 months old may have only 50% viability, meaning only half the cells are still alive and can be made active again. A yeast starter will help grow new healthy, vital cells.
To make starters you need a large (glass) container, most use glass erlenmeyer flasks, they are flat bottomed. Also a stir plate with a stir bar (or an orbital shaker) to propagate the yeast, as it needs a continuous supply of oxygen (air).
Having pure O2 on hand can be useful too, and you need it to oxygenate your batches anyway.
For 20 gallon batches, 5
gallon liter erlenmeyers are probably the most useful, and you may need 2 or more of them, depending on how many times you brew. That also means 2 or more stirrers (or shakers) if you want to run faster production. But you could do it with multiple 2 liter flasks and cheap or homebuilt stirrers (or one or 2 large orbital shakers).
You also need a fridge to cold crash the starters.
You need a large (stainless) pot/kettle with a well fitting lid to prepare several liters (gallons) of starter wort.
Whenever handling yeast you need to practice good sanitation methods or you'll be raising 'bugs' at the same or faster rate.
