There's no limit regarding VMs on an ESXi host, other than what the host can handle (CPU, memory, ...).
Did the issue occur out of the blue, or has something been changed on the Mac, e.g. updates, ...?
Did you check whether any log files are created which contain details about the error?
Thanks for your reply.
Unfortunately, I could not find the cause in the log files of VMware Fusion. But I recreated a new VM by keeping only the VMDK file (therefore recreating all other files) and voila, this newly created VM could be uploaded to vCenter.
Strange, but it worked.