How to get Azure MFA for 1:12 ratio external users?

A client assigned me to find IDM/IAM solutions including SSO and MFA for both their ~250 internal as 3.000 external users, i.e. a 1 to 12 ratio.

We looked into the Azure MFA pricing model since it's quite an impact to purchase 3250 per-user licenses instead of just the 250 internal users. On the other side, a per-authentication model (10 per bundle) leads to unpredictable cost, especially if some hack tool generates a gazillion failed attempts.

So our main question is how to properly license Azure MFA for 250 internal users and 3000 external users? The underlying questions we have:

1. Does the 1 user : 5 licenses ratio in an Azure B2B scenario only apply to Azure AD Premium or also to Azure MFA standalone?
2. Is there a possibility to mix a per-user license model for the 250 internal users and a different model for the 3.000 external users? (maybe even the 'free' MFA for Office 365 when they go there)
3. What does Microsoft do/bill in a per-authentication model if e.g. a robot generates a substancial amount of (failed) MFA transactions?
4. Any chance there's some sort of External Connector cloud variant for Azure MFA?

Thanks for all your thoughts and answers!

With kind regards,

Mike