I have never seen a welder fry an alternator. Seems to me I have seen ground wire get hot,,,, best I can recall when someone ground to body, work on frame etc but its been so long I cant remember details.
As Bob said,, at least a thoousand and this is a real number and maybe even light at that, even only once a week over 30 yrs would be over that, on occasions did one every day, sometimes worked off of truck bed several times a day.

I am sure it has happened somewhere at some time, at least something but never seen it personally, don't know any reliable first hand events either as I can recall and actually see more possibility of problems disconnecting etc and certainly giving more of a feeling of security than any real benifit.

I have heard stories,,, one remarkable one comes to mind on a forum, maybe here guy says he blew up his battery, cooked the alt and blew out all the light bulbs on his pickup truck and was here to let eveyone know,,, be sure to unhook battery, I said,,, send pics,,, I wanna see this, ha

These trucks are used as work platforms by welders by the thousands every day. If it was a problem we would hear about it.