Bob,
Well there you go! You asked a simple question, but the answers you got took it a step further and may scare you more than a little.
The simplest answer would be: if you're planning on shooting just for fun, it really doesn't matter. Put 10 extra holes in your plate, then go for it.
The reason behind us saying we shoot so many additional shots "off target" are multiple.
In the beginning of a Card it's first to foul a clean barrel, then to study what affects the current conditions are causing before we get serious. Right there you'll probably use a minimum of the 9 shots Keith mentioned, but usually more.
Then, while shooting a card it's usually because of a condition change. Add to that the fact that several of the top shooters will also go off to the side Sighters and shoot 2 to 3 more shots every time they change a line.
So far what I've mentioned would take the 25 for score shots, plus another 10 or more before even starting the card, and then add another 12 to 15 shots into the side Sighters when changing lines, or testing a sudden condition change. There's your 50 shots per Card.
It can all add up quickly, hence Ken's and Tony's comments about allowing for a box per Card, plus, whether you'll be shooting 3-Cards or 6, to be safe add an extra box of ammo when preparing for the match. And it wouldn't hurt to throw in a second extra box just in case things really get interesting during the match.
Personally, I don't think I ever shot 50 rounds per Card, but I sure did come close on more than a few occasions. My average was probably closer to 35 to 40 shots per Card over my 20 years of competing in rimfire.
But, what do I know.
Dave Shattuck