I agree, that is a good number of t-shirts to pack. Although, depending on the ship, some have self service laundry, if that is the case I can lower that count and do just one load of wash and not return home with everything in need of cleaning.
If there's air travel involved my preference would be to take half that many (and half of everything else as well) and use the ship's laundry (either self-service or full service) on the third or fourth day. For a 7-day cruise this should make it possible to take only a rollaboard and a small underseat bag, eliminating baggage charges. This would make the laundry vs. baggage charges a "wash" and would allow the person to travel lighter.
Don't say it can't be done. We were recently on a TA with two fellow passengers who are airline flight attendants. They each did it with a rollaboard and it was a much longer cruise.
The most dangerous man in society is the man who has nothing left to lose. -- Saul Bellow