I think a lot of this depends on whether you get visa-free access or whether you'll have to buy a visa. If visa-free, all you usually end up with is a little stamp. If you have to buy a visa, they usually come as full-page stickers. But assuming you don't cross borders repeatedly, 9 pages should be sufficient.
If crossing between Tanzania and Malawi, DO NOT change money with the MONSTERS on the Songwe bridge, no matter how anxious you are to be rid of your Tsh or your kwacha. DO NOT take out any money. Those people are thieves and will steal your money in broad daylight if given half the chance. They will surround you and distract you while one person makes off with your money. They'll also try to intimidate you by sheer strength of numbers and by shouting at you. If you're caught in such a situation, ignore everyone and head straight to the border police. You must also be able to identify at least one of the thieves.
Appalling behavior really. I've crossed numerous African borders. In fact, I live at the Kenyan-Ugandan border but I've never experienced anything quite like the Tz-Malawi border. Shame, given how much I love Tz (the first African country I ever visited).