I want players to have 5 aviable slots by default when they craft toolbelt

you should be able to do this with a datapack or crafttweaker script. just make a replacement recipe that has the right NBT tag in the result item.

thanks, figured it out, if anyone needs it, just add nbt in belt.json recipe like this:
{
"type": "forge:conditional",
"recipes": [
{
"conditions": [
{
"type": "toolbelt:enable_normal_crafting"
}
],
"recipe": {
"type": "minecraft:crafting_shaped",
"pattern": [
"sls",
"l l",
"lil"
],
"key": {
"s": {
"item": "minecraft:string"
},
"l": {
"item": "minecraft:leather"
},
"i": {
"item": "minecraft:iron_ingot"
}
},
"result": {
"item": "toolbelt:belt",
"nbt": {
"Size": 4
}
}
}
}
]
}

okay but does this let you use MORE than 9 slots? because thats what id like