MicheleM,
First of all, if you create a VM for a SMS than it really does not matter.
You can start with 16GB and 4 cores, when you see it is running short of memory you add some, same for the cores.
The point being that it differs per environment, when you log every rule and you have 1Gb data per gateway per sec, this will put a high load on the system.
Your question is there a difference between 10 clusters and 20 gateways, the answer is yes, in most clusters only 1 member is handling traffic and sending log's 20 gateways will have 20 streams of logs, you will also have more policies to maintain.
Regards, Maarten