I have fresh ripe peaches and plan to make a cobbler. Recipe calls for 20 ounces of frozen sliced peaches, how many fresh (medium sized) should I use?
If the problem is that you don't have a scale in your kitchen, (which for some reason is common in the USA, I grew up in a house like that) then get a quart measuring cup and hack up fresh peaches until you hit (or exceed, I agree with the comment) 2-1/2 cups - because 20 ounces by weight of frozen peaches is pretty near to 20 ounces by volume of fresh peaches and juice, if cut and packed so there's not much air space. If leaving air space it might be full quart of less packed down/cut up peaches.
Also, put a kitchen scale on your gift list for whatever gift giving occasions (including gifting yourself) might be coming up, and correct that situation.
If you have a scale, remove pits and weigh 20 ounces of pitted peaches.
How many medium peaches gets caught up in "what's a medium peach" but seems to be 4 according to one definition of "medium" (5.3 ounces, I'm going to guesstimate the pit as 0.3 so it works out, and I'd choose 5 to be sure there was plenty, since @mosjafj is correct in the comments. And pits might be heavier than that.)