Ceiling and roof sagging in ranch house
Will adding more 2x10’s to the 4 existing help and if so how many in total would be required ?


This is very common for older stick framed homes that were not engineered and the code very minimal. The reason is obvious, it is not constructed strong enough

What you did with the 2x10's is called 'sistering' or 'sistered.' The way that you did it does not add strength to the roof structure as a whole, as you have learned.


  Or is there a better solution?


Answered directly and simply

Yes

Answered more fully aka 'How' or 'What' is a better solution

It's complicated. It depends on the particulars of your roof framing, the wall layout of your home, and other issues. It involves not just the old framing member sizes, locations, wood species etc, it also includes the connections of the members and other things like constructability issues (how to get members into place at minimal cost)

Generally this is a project for a structural engineer. I'm guessing that you may note like that idea, for reasons. But there is no answers to be found here, from friends, and probably not from contractors, it's simply too complicated an issue.  

There is ONE option that might be plausible for your situation: you can make the ridge into a post and beam system. This reduces the rafter spans in half. You'd need a beam of course, and posts, and footings (the existing foundation also surely not able to take these new point loads)