M advice to CEOs in 2018 is to ask your CIO about Serverless. If they dismiss it then fire them.
You'll thank me later.
#facepalm.
If you must, as a rule of thumb, for an estate providing functions to others then ... one ops person + $300k p.a. for every 10M active users. This is for everything, electricity, buildings insurance, software, tin, monitoring etc.
Then don't. Have a policy of public serverless first, then public cloud (containers etc) if you absolutely can't in a specific case. After five years, you'll have enough experience to laugh at your home grown plans.