We have been informed this week by Support that it is actually the number of emails delivered to the record, not by the sends.
This is from the product team - "The marketing communication limit keeps track of how many emails a lead has received per day, and this is what is used to enforce the communication limit. This table is updated only when an email has been delivered"
It is also worth noting that if you are carrying out large or complex sends, then you have to wait for the first send to write to the back-end tables of Marketo before the communication limits will block anymore sends.
So if you send 2 emails 15 minutes apart with a potential overlap you should always include an exclusion filter of 'Was not sent email today' for example to cover yourselves especially if you feel you will hit soft bounces (that are tried again over 24 hours to deliver) or slow send times due to complexity of smart lists.