There is a similar question on this site, Are there any sites that let me book 7 or more multi-city flights?.
But, it's from 2013. And lists just one site - https://airtreks.com, that gives an estimate. For example: 1. https://tripplanner.airtreks.com/estimate/934439 2. https://tripplanner.airtreks.com/estimate/934441 3. https://tripplanner.airtreks.com/estimate/934450 4. https://tripplanner.airtreks.com/estimate/934452
We're in 2020 now. In the last 7 years, i am sure some improvements must have been made in this regard. At least, i have hope.
I checked itamatrix, google flights and skyscanner.