I understand that Google Scholar offers this service, but it has limitations and does not offer the correct number of co-authors. Are there any other websites or tools to retrieve the total count of co-authors?
A very similar question was previously discussed here. I suggest you use the bibliographic database Scopus by Elsevier, which is an established reference (at least where I live). To update the answer by user "gerrit" (Scopus has changed a bit in the last 8 years), search for the name of the author of interest and click on "Co-Authors":
Note: Scopus is a paid service by Elsevier, so the free version you can access without registering is quite limited. Ask the support at your university library to check whether you have access to the paid version.