Unfortunately the answer for this is it depends. It depends on which security services you have enabled like AV, Antispam, Greymail, AMP, DLP etc and how many message and content filter are being used in your environement.
To give some numbers, on our peak days we had about 90 Million message attempts for our domains and this caused serious delays for at this time 4 virtual v600 ESA. In the meantime we have increased our capacity to 6 gateways but have also added many more plausibility checks for incoming emails.
I think if i where you I would first avoid having a single point of failure first by adding a second either hardware or 2 virtual gateways to your mix.
Lets assume we calculate based on our worst ever day.
90'000'000 / 4 = 22.500'000 connection attempts per gateway
based on SBSR and SDR normally cutting out 50% you end up at 11'250'000 message a day which need to be processed. We had all security features enabled and we had an about 6 h delay that day in processing incoming emails. For ease of use that would means 11'250'000 / 24 to get an hourly rate = 486'750 messages per hour and we did not have enough cpu power to process this. Realistically I think we can cope with 300'000 messages an hour per gateway to stay within our service levels.
Looking back at your 6 Mio messages a day number you can see that you should be able to do it but are already in the 70% load range, so while it could work you will have serious issues should your single ESA go down or be blacklisted.
I hope that helps
-Marc
