Is there a rule for giving tips?

Hello! I am about to set off for Rome these days for I have booked a cruise with Costa Magica and it starts from Rome. Anyway my cruising will take about 8 days I guess and I now wonder what would be the approximate money amount to bring for tipping and is there a rule for giving tips on Costa Magica anyway? Thanks!

Sure there is some rule like for giving tips and you could get precisely informed on this at your TA but I am sure you are about to bring extra money for giving tips is very common on each cruise ships involving Costa Magica also. I also had the same cruise but it was about 4 years ago and I cant exactly recall what the tip was about but I surely gave to those butlers and steward lets say about 1 or 2$ upon each delivery plus I tipped my martyr with 10$ before disembarkation.