If you have 4 W-2 forms from the same employer for example, you have to choose '1' as quantity. The Quantity itself on Step 3 - Income documents is asking you to choose the number of income documents you have from different employers.
If you worked for Company A and Company B and got a W-2 form from each - you have to choose '2' as quantity. The same goes for the 1042-S form.
For the 1099 forms though, you have to choose based on the number of transactions you have listed on your income form received. If you had 10 transactions from Tesla and 10 transactions from Apple, you can choose 2 as quantity and just write in box 1 'Description of Property' of the 1099-B populated 'Tesla 10', 'Apple 10'.