Shopify shows this in Admin, under Analytics, then Reports, then Total sales by product. Add Product SKU as a dimension, filter to the SKU you need, and set a custom date range. The Quantity column then shows total units sold for that SKU across the whole period, net of returns.

This works on any Shopify plan, including Basic, since Shopify's Reports section now covers all plans rather than starting at the Shopify plan tier. The quantity shown is a net figure: sold items minus returned items, so cancellations and returns are already backed out. One thing to watch is that the default "Total sales by product" view groups by product, not variant, so if the item has multiple variants under one product, add SKU as a dimension before reading the number, or the total will include every variant, not just the one you're checking.

Once the SKU and date range are set, this becomes a one-off lookup rather than something built to reuse. For checking one SKU on a recurring basis, monthly or weekly, Report Pundit's Sales by Variant report keeps SKU and net quantity as default columns, so there's no dimension to add each time, and the same date range can be saved and re-run or scheduled to arrive by email instead of being rebuilt in Shopify's dashboard each time.