How to disable/remove/hide Quantity

You can use these CSS code to Website Tools > Custom CSS to disable/or hide/or remove Quantity
#1. All Products

div.product-quantity-input {
    display: none !important;
}

#2. Products belong specific Shop Page Only
First, find Shop Page ID. Use below too

In my example, it is: #collection-667f8bab720ee26389417857

Next, replace # with dot body. so new ID will be
body.collection-667f8bab720ee26389417857

Next, use CSS like this

body.collection-667f8bab720ee26389417857 div.product-quantity-input {
    display: none !important;
}

#3. Product belongs specific Category
We can’t target tag, so you need to create a tag with name: no quantity and assign it for products.

Next, use CSS code like this

/* hide quantity */
.tag-no-quantity .product-quantity-input {
    display: none !important;
}