There are mathematicians/statisticans out there that make a living by telling you exactly the number of patients per protocol you need to address a specific question. The number depends on your device, the type of intervention, comparators, indications, ... Noone can give you a number without much more details (more than you would want to share here).
Best guess: start with a low number of patients in a pilot trial (first in man, explorative trial). Then you can use the data gathered to get a better idea how many patients you need for your confirmatory trial, and what endpoints to define.
The NBs evaluator will want to see an explanation/calculation of your study population. By the way, you can ask your NB for a project meeting and discuss the studies you are planning. They cannot counsel you, but they might tell you when you're far off the mark.