I'm not sure how you'd do it based on a question asked during registration, but the supplier profile questionnaire itself could have the "Are you a food supplier?" question. You'd then build the profile out with content that is visible based on that response.
For an example of this, take a look at the "Are you a Woman-Owned Business?" question (and others similar to it) in the standard profile: if the supplier answers "Yes," a conditional question comes up asking them to upload a certificate. You'd use this same functionality, except you'd have two sets of conditional items: one for the Yes response, one for the No.
That seems like the simplest way to handle it, though it's probably not the only way.
To answer your original question, in On Demand there is a limit of 400 "questions" that can be created. Questions can be actual questions, or more broadly, individual pieces of information that can be asked (i.e. dates, drop downs, text, etc.)
No, there is only one supplier profile allowed, but as answered earlier in this thread you can use conditions to display/hide certain questions depending upon answers to previous questions.