Hi Ian,
First you need to enable “Per product” checkbox in “WooCommerce > Settings > Maximum Products per User > Quantities”. After that you have two options:
1. Go to some product’s admin edit page, locate the “Maximum Products per User” meta box and set max quantity there.
2. Use new “Formula” feature that we’ve added just yesterday: go to “WooCommerce > Settings > Maximum Products per User > Formula”, enable the section and set “Formula” to something like this:
[alg_wc_mppu_max_qty max_qty="10" product_id="100"]
Here you need to replace 100
with your actual product ID(s) and 10
with desired max quantity.
Hope that helps and please let me know if you have any questions.