9x9 pan yields 12 small brownies or 9 medium sized brownies.
If recipe is doubled, a 13x9 pan yields 24 small or 12 large brownies (large ones are in my photos). I double the recipe a lot (we love brownies!). If you double recipe, it will fill a 9x13 pan with no trouble. Continue to check for "done-ness" beginning around the 20 minute mark.
JB's Best Brownies https://www.thegratefulgirlcooks.com/jbs-best-brownies/