Is there any upper limit to the number of guest OSs that can be created when using the WindowsServer2012R2_Standard image in Dedicated Hypervisor(Hyper-V)?

You can create guest OSs under following technical restrictions and license constraints.

▼ Technical restrictions
As shown below, Microsoft has defined the number of virtual machines that can run per Hyper-V is 1024.
https://technet.microsoft.com/en-us/library/jj680093(v=ws.11).aspx

▼ License restriction
If you use our guest image, there is no restriction on the number of units by license.
License will be allocated according to the number of virtual machines that have been started on a particular host more than once during the current month and will be charged.

If you wish to apply for your own licenses, please contact your license supplier.