What number of clicks for SIS?
 

Shifters will have one less click/detent than the number of gears, you have to count the starting position as 1. Generally Shimano shifters will index one gear past with varying degreees of success.

I think I've only ever setup one bike like that and had it shift perfect, was a 7spd XT thumb shifter running an 8spd cassette if I recall.

You're right about the XT- and perhaps Deore-level thumb shifters, even the 6s version had 7 solid detents.

But I don't think there are any other examples of "extra-click" SIS shifters, especially among road shifters.

My CX race bike has run 7s XT levers with an 8s cassette for over two decades, and I did slightly modify the spool groove diameter for better indexing with an 8s XTR cassette, and it's been race-day reliable over all of these years.

I think it's limited to thumbie and downtube levers with a friction option. The lever just sort of travels past where it should and sticks at the low derailleur stop. I'm not a big fan as it rarely works great in practice.

^^^ I bought a used bike so equipped. '89 Trek 800, which shipped as an 18-speed with a freewheel and thumbies. PO took it to a bike shop to replace the rear wheel, and they installed one not only with a 7-speed cassette, but 135-mm OLD. They just spread the triangle to force the wheel on. And indeed, it did just BARELY managed to hit on all seven with the "ghost" position, albeit not very well. (Okay, not well at all...) Fortunately I found 130-mm wheel and 6-speed freewheel for it; works MUCH better with the proper kit.

Definitely a ghost bonus shift on many SIS down tube shifters , even on the 9sp and 10sp, but counting on it for a n+1 setup would be highly optimistic and invites a R shifter failure in the future.