I think every office would have a handy pair of scissors.

My brother used to always cut pizza with scissors for my nephew when he was a toddler/preschooler. He/we always had knives and a pizza cutter, but he insisted that it was easier to cut with scissors.
If buying pizza for a party or pre-planned whole group lunch or something, I would definitely plan for 2+ slices/ person. I wouldn't look at it that way if I'm just ordering a little extra of my lunch to share. I'm not providing a whole meal for everyone, just a little something extra. I do think it's rude to take more than one of anything when offered, unless specifically told (without asking) that I should take more. Reminds me a bit of trick or treating. If a bowl of candy is left out on the porch or someone holds the bowl out for the kid to grab the candy themself and doesn't specify how much to get, you grab one piece.
Speaking of pizza ettiquette though, I've got a story. Last year, DD's 1st grade class went on a field trip to Chuck E Cheese and I was a chaperone. The kids normally have 2 snacks at school (as well as lunch of course). We were told that snacks and lunch would be provided for the students, so not to bring anything. Their provided snack was a single nickel size taste of cotton candy. Their lunch was this:
It was seriously about 3 bites of food. One slice per kid, nothing else. Meanwhile, the teachers are all sitting in their own booth with huge pizzas with normal size slices, salads, and breadsticks.
I watched as one kid went up and told a teacher he was still hungry. The teacher replied, "Oh, didn't you get your pizza over there?" He told her that yes, he did, but the kids were only given one really small slice and everyone was still hungry. The teacher told him, "Well, that's all there is then." I then spoke up and said I was going to buy a pizza because the kids in my group were still hungry too and he could have a slice, only to be told by the teachers that I can't buy a pizza for some kids without buying for all. I was a little frustrated by this point, but replied, "Fine, I'll spend $100 buying pizza for everyone. I'd rather do that than let kids go hungry." A couple other parents overheard and immediately offered up some money. Then we were told by the teachers, "Well, it's too late. The kids' lunch break is about over and the pizza won't be ready in time so the kids won't be able to eat anything you order." I ended up stopping at McDonalds to get DD some food on the way home. I felt terrible for all the other kids whose parents didn't drive them and would have to wait until they got home to eat. It blew my mind that teachers would continue stuffing their faces while essentially telling 75 hungry kids that they couldn't have anything else. Also the fact that they considered a 2.5" wide slice of pizza and a taste of cotton candy an adequate school day worth of food for kids.
