I'm creating a saved search to get the average quantity for all sales orders. My results are as follows. But when I run it, if there are multiple items in a sales order, the Internal ID is counted as many as the item lines in the SO, which shouldn't be the case since it is one SO only.

internalid is the internal id of the sales order, and since you are getting the lines, this would be expected. It's going to reutrn the same internalid (of the sales order) for every line

Is there a workaround for this using a formula in the saved search itself? Should I create a stored header level total quantity field then (which I am trying to avoid)?

I dont really understand what your summary columns are trying to accomplish, what internalid are you trying to count?

Internal ID of the sale prder

Use a Forumla (numeric) Maximum, COUNT(DISTINCT({internalid))

Thank you! I updated as well the formulas of the other fields and it worked, thanks to that eye-opener