I remember (dimly) having two horses and one vet, when all I had to do was make an appointment in the spring and another in the fall. The only thinking required was remembering how to write out a check. But in Herd Life[tm]? No more. Especially when Herd Life[tm] has beasts scattered about the landscape at different farms in different counties, which means (predictably) different veterinary practices. (With, if one is really unlucky, different management recommendations.)
But now I have supplemented this spring's mare spreadsheet with a generic template for adult horses, and an additional "calculator" page for foals (put in the birthdate, get back dates for recommended shots and worming for the first year). Now I can be done with fretting over the state of my horse records, and return to fretting over the state of my horsekeeping (floats! floats for all!). Not to mention the eternal question of where to find a vet that makes routine farm calls on weekends, apparently a vanishing species in my area.
To answer my initial question, I have decided that it's a herd when you can no longer fit all your horses into one trailer and get out of Dodge. (Which led to some hard thinking two wildfire seasons ago, when it was looking like evacuating was a good idea. Fortunately the fires didn't come close enough for me to be forced to decide who to drop off the life raft...) Which means that happened three years ago, when Recap was born. Silly me for not acquiring a 4-horse trailer to start with!