Obviously the risk is there, but given that he has provided very personal details I can't imagine that he is not keeping track of all of these incredibly closely. I have about the same number of cards and it usually isn't that difficult to use a simple Excel spreadsheet or some other paid monitoring service to be alerted for unusual activity, especially if you do it on a regular basis. Given that he is a financial advisor of some sort, he could possibly be paid by his clients to monitor hundreds of their accounts too.

I think I remember seeing a commercial on TV sometime back where the guy says "My name is xxx and my social is yyy", and my identity is secure. The more people know your identity, in general (but not always) the less likely it is to be stolen. A few year ago, I charged my card at a restaurant in a foreign country and lo and behold a few months later I get a call from my credit card company telling me that someone is trying to charge the card again for $900. I had the card with me but they retained my number at the restaurant. In my multi-decade credit history, this is the only incident of the kind that has occurred, and the card company declined the charge. Just be careful to keep track of all your cards and you will minimize the risk - though you can never eliminate it completely.