I have multiple containers with blobs(.xlsx format) in it, how can I use one azure search service so that on each rest api call I can get sum of count of a column from each blob or all blobs ...
There is not an out-of-the-box way to do this.
A few suggestions:
If you are reading blobs using the Azure blob SDK (not using built-in indexers), you would have to extract the Excel data and compute the count (examples of how to get the Excel data with .NET are here)
If you are using pull blob indexers you can either:
Use Azure AI Document Intelligence (Form Recognizer) custom skill to get the data from tabular form and then in the custom skill do some calculations for the count.
From a custom skill as well, get the data using a library that reads from Excel and then index.
Here is a repo of Power Skills (custom skills applied to different use cases) in case you need examples of how to use them and the skillset official documentation