2 at this point.
Only 1 was included with the boxful of pieces (the loco was completely broken down, even the siderods were removed) that was sent back to him by that large, expensive but to be kept nameless, well-known VT DCC site. Their response was that the loco was 'broken.' So I user a pair of screws from one of mine to put it back together and make sure it ran OK in DC before I did the install. Considering that the 1 screw they did return looked like it has been gnawed on by a steel toothed rat I guess I'm not surprised they felt the loco was broken.
But it seems to me that I should have a few spares, considering that I'm likely to have a few more Mikado installs coming my way the next few months which is why I was asking for a true size so I could see if I could find an acceptable substitute.