I was going to number them by first name, but then I wondered if it is better to number by last name. I really don't know.
I number my students by their last name. That way, the numbers are in the same order in my grade book and attendance book as well. Any correspondence I receive from the office (such as bus lists, lunch numbers, etc.) are in that order as well. That's just how I do it, but I'm sure it could work with first names as well.
Sorry, but first name here. That's what I call them by so I guess it's easier to work with when noting their grades. I guess whatever works for you as long as you don't call them something rude! I did find something new this year that I tried and has really worked well. I wrote right on their desk with a permanent marker the number 1, 2, 3, or 4. Now I say things like: "Number ones, go get the books for your group. Number 3's, you are in charge of collecting the work and making sure names are on papers." I like it a lot. The only problem is that I've already moved some people away from others, and it messed up the numbers. So now I have to remember to bring hairspray to school to get the marker off and renumber them!
I ended up going by last name. Really, I prefer first since I've done that when I taught kinder, but, I figured since I'm in a new grade I might as well try something different. Also, it was just easier too. All I had to do was look at my class list and give them their number. I'll see how it goes this year.
I number by 1st name, because usually that is what is on their papers already. I've never done it by last name but seems like most people so far said last, maybe I should try that this year.