12 was probably refering to a maximum recommended number of lookup columns in a list. It's a little different from the number of items you can select in a multilookup column.
The number of items you can select in a multi-lookup column is dynamic.
From Column limits:
The sum of all columns in a SharePoint list cannot exceed 8,000 bytes.
As you can imagine, 8KB is not a lot of data. So you can probably get away with adding 200-300 items in a multilookup field. The maximum I was able to add was ~520-540 items.
I should warn you that adding this many lookup items is going to be really bad for performance if you are going to show all these items in a SharePoint List view. As a workaround, you can just hide these lookup columns from a list. As long as you stick to 5-15 items in a lookup column, you should be fine.