Once I was working a fairly large maple yard tree on a bowl commission that was used as a tree fort. I should have aborted the job immediately, as I removed over 50 nails before I finally quit harvesting.
I estimated that I only worked half the area with nails, so 100 nails is certainly my record for a tree. Luckily I found most of them with a metal detector and dug them out, but these days as soon as I start finding nails out comes the splitting maul making firewood.
With nails I generally try to have a light enough touch that I stop and prevent extensive damage as soon as I hit one, but I have had trees full of concrete that ground off the cutters in a heartbeat.
That is why I always bring a bag of chains to the site and do my sharpening at the shop with a grinder.