Note: Adding more than 10 refiners may impact search performance
Matthew McDermott recently asked if the limitation for 20 refiner categories is a boundary or if it can be changed.
Using the refiner configuration on a search page as seen above you see that you cannot add more than 20 refiners using this dialog. There are at least three ways in which you can overcome this.
Open up dev tools in your favorite browser and remove the “disabled” attribute of the Add button. Below is the same image as above, but with the button enabled.
Export the refinement web part, open it up in your favorite editor and manually edit the SelectedRefinementControlsJson property, then import the web part back into the page.
Use PowerShell or code to check out the page, get the web part and modify the property in option 2. Much like my post on turning off duplicate trimming.