Figured it out for you.
"produces": [
{
"item": "atlas_expo:upgrades:blue_clay_window_frame"
},
{
"item": "atlas_expo:upgrades:blue_clay_window_frame"
},
{
"item": "atlas_expo:upgrades:blue_clay_window_frame"
}
]
Place the item as many times as you want with commas after them in the recipe. This is from my mod, so don’t copy it directly.