Atta, since actual data processing is performed by the proxy servers, not Veeam B&R server itself, Veeam B&R server's resources are not that important. I suggest reviewing the corresponding user guide section (p.14) for better understanding of Veeam B&R deployment architecture and the proxy server role and specifications.
Assuming that these 40 VMs are spread across your customers, I suppose you will be able to successfully back them up using onhost backup mode.
Regarding SQL Server, please review this post
(still applicable to the latest version), should address your concern.
Hope this helps.