Data type of list and quantity

I am very new to and I am not even sure how to search for what I am asking. Please forgive me if this has already been asked and answered.

I have 3 data types.

The order will contain 1 or many layouts
The layout will contain 1 or many Fixtures, but i want to also be able to have many of the same fixture. So a user would select a fixture to add to the layout and then type in a qty of that fixture type they want to include on the layout.

I can create a field type that is a list of another datatype. I am curious how I could store the qty related to the fixtures that got selected to be added to the layout.

Not sure if this is the best way, but I was going to create another datatype of selectedFixture, and with two fields, one being the link to a fixture and the other being the quantity of that selected fixture. Then in layout datatype , have a list of selectedfixtures.

Just curious if there is a better way to do this than what I am thinking.