I keep 5 in the tube and 5 on the stock in one of those nylon sleeves. There are a total of 9 3" Remington 00 buckshot and 1 2 3/4" 000 federal. The reasoning is this; I picked the Remington because at 15-20 feet it had a wider pattern out of my gun. It throws an approximately 10" pattern at those distances. I tested federal 2 3/4 00 as well as 000. The federal 00 had a very tight pattern at close range. About 4" or less. I thought this to be too tight. I also discovered that my gun hates to feed the 5 th 3" she'll but it will reliably feed 2 3/4" shells every day of the week. The 000 buckshot threw about a 6" pattern so I put that in the last tube position. I hope to God that 1) I'm never in a position where I need this and 2) if I do I hope to settle things with a lot fewer than ten rounds. I really don't have the training for a protracted gunfight. It's not something I fantasize about or think I would be good at.
That being said it seems to me that a strength of the pump shotgun is the ability to top it off as you go. I keep an old shoulder bag with loose shells just in case.