Hi @kumarH
There are several factors which could influence the number of spines as well as the type of spines:
1. Total throughput (bandwidth)
2. Number of Leafs in each pod (this actually influence the total throughput in the fabric, so back to #1)
3. Level of redundancy and resiliency
Regarding your second query, this might help you: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/4-x/verified-scalability/Cisco-ACI-Verified-Scalability-Guide-412.html
The number of leafs in a pod/fabric is not dependent of number of spines, but rather the total number of APICs.
Also important note to remember: Maximum spines per pod: 6.
Stay safe,
Sergiu