Looking to add quantity of all records from all tables in one place

The idea is that on a separate table, I want to calculate sell-through rate. So at any given time, I want to pull up this table, and see how many records I have across all my inventory tables. These are organized onto separate tables based on category.

In the next column, I want to be able to calculate the quantity of sales from a particular month within my sales table. My sales table already records the month each sale was made, so I just want the total quantity from the particular month on my sell-through rate table.

Any ideas?