Create a new field-based view sorted by Unsorted (i.e. NO SORT) to which you’ll add a block display (no need for a page display here).

Add the field which contains numerical data and values. Remove or hide all other fields.

Add the required filters as needed (to fetch the needed content and parameters)

In the Advanced section, check Use aggregation. Save.

Back in the FIELDS section, click Aggregation settings and set the type to SUM. Save. The field should now look like: SUM(Content: Number of Units (Stock Availability)) | Aggregation settings

Click SUM(Content : Available spaces), click on Rewrite results and check Rewrite the output of this field using the available replacement tokens.

Example in this case:

  • Sum Total highlighted as heading text.
  • Usual text label below it Save you view.

Go to the blocks admin page and configure your block.