How much is this valid to use Cisco Core Switches e.g. 3750 or 4500 series for CCTV networks with 20 to 300 IP cameras few servers and clients?

Using a core switch is very highly recommended when using large number of IP cameras.

The main issue that is faced is the high bandwidth generated by the IP cameras video streams while communicating with the NVRs or VMS servers and also the servers communicating with the clients.

By enabling the multicast routing on a core switch clients and servers can subscribe to video streaming groups created on the core switch which means that a single video stream can be used by multiple requesting machines (Client/Server) instead of sending multiple video streams for each requesting machine which will dramatically increase the BW over the network.

when is comes to choosing the correct core switch, this would be specific to the project size and expected BW by the cameras and the network usage as well. As each core switch model have certain specs in regards to the BW.

In regards to the servers hodling the VMS, there is in the market a lot of NVR vendors providing diff. specs as some NVRs can handle upto200 IP cameras on a single machine. Again this also depends on the cameras models, Megapixels and expected BW by those cameras. 

Well it all about the client requirement from that network! if the client is asking to connect different networks or LANs a core switch is required mostly. The BW playing a big role but not for the mentioned number of cameras.

For the recording servers, yes you need the best server in the market to preform properly since there are big data processing happens on these servers, sure that depending on the software application.