"fall in" and the troops come in....or like with was done in class; "anderson" "here" "bailey" "here" "jonathon"......*crickets*...."Jonathon"........Nope not around. Or, if it was an entire town, by a town census type counting. There are many ways this could be done, but I wasn't there to know exactly how. My best guess, since I am fairly sure Willie was correct in that he was a soldier or whatever they called it back then, that it was a more militant style of counting. Sorry I can't be more help here. I don't think the Bible specifies the hows exactly here, but I imagine, with the technology available back then, this process took a day or three to complete.
Although, I don't think the exact method is as important as the meaning of the writing, I was not necessarily searching for the same understanding, when I searched, as you were when you searched.
Peace!!