Why are you doubling for redundancy? Something to remember; 9 switches max per stack. Interconnecting stacks may require more than 1 Gbps link, so you may have to etherchannel multiple links to multiple stacks. Now the four ports you have designated for inter-connectivity may not be enough! You might want to look at a chassis based switch too. Depends on your requirements.

Yes, if 1,300 hosts only need one port, then using the 48 port 3750 switches, you need a minimum of 28. However, if the physical topology doesn't have all the host cables terminate in one spot, you'll need to allow for fractional usage of switches. E.g. 90 hosts that would only require two 48 ports switches, would need X number of switches if they terminate in X different closets.

You're also going to need to use ports for interconnecting switches that are not part of the same stack. The SFP ports are available, but there may not be enough or you may want to use some of the pure copper ports, if possible, to reduce cost. This might increase the number of switches necessary.

Not clear why you're doubling the ports for redundancy unless you plan to multihome or NIC team all hosts. If temporary loss of a switch's connected hosts is permissible (e.g. user host vs. server hosts), then you might just have one spare switch on hand to replace a failed switch.