I got this from Nathalie DuPree years ago. It's the best iced tea EVER, and keeps for weeks. The "20" servings is a guess. It depends on the size of your glass; how strong you like your tea; how much ice you use, etc.
I use a little tea kettle for this. I put water in the kettle, and then add the tea bags and get the water to boiling. Take the kettle off the heat, and add the baking soda (it will foam over so make sure it's not over the burner or too full). Let it set and cool off.
I put mine in a pitcher in the fridge (some times with ice; some times just as is). It's almost black in color, and very strong. You need maybe 1 tbs. in a glass of water/ice, depending on how strong you want your tea. It keeps, without having that nasty taste, for well over a week (i've actually kept it almost 2 weeks and it's still delicious). Just remember it's VERY strong, and you need very, very little added to your water.