Hi Eddie,
Sounds like you are in need of attributes. Help on setting up attributes can be found here https://www.zen-cart.com/tutorials/index.php?article=57.
You can set up an Option name called "select" from the option name manager under the tools tab in your admin. From there using the option value manager you would add 3 options (for this example) and they would be 100pcs, 200pcs and 300pcs.
Navigate to the category that your product is in from the admin. There is a series of buttons next to each product. You are looking for the black "A". Give it a click and then you can add your "Select" option and any or all of the 3 values that you have assigned to the select option.
You can give a different price, weight and sort for each of the options you add.
The cool thing is that if you wish to offer many products in 100, 200, 300 . . . quantities you may "re-use" your "Select" and its values for all of the products. Each time you do you can choose a different price, weight and sort for each.
Take your time with attributes. It can be quite overwhelming at first but they will quickly grow on you and will become much easier to use.
Good Luck,
John