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Should be 2 on the driveside, one on the nds for 68mm shell, one on the driveside for 73mm shell and subtract 1 driveside spacer if you are using a bb mounted front mech or chain guide.
Or at least that used to be how it was.
Think the road bbs are narrower cups to allow for smaller q factor. There’s no need to build in the 73mm option on them as that’s a mtb thing as are bb mounted chain guides and mechs so you could be right that the threading on the cups will be shallower.
I guess you could look at it like the road bbs are 68mm wide, the mtb ones are 75.5mm that you space down.
M_V
@M_V Thanks. Opted for a XT BB. However I have only installed 1 spacer on on each side. Took it for a test and shifting is ok. Noticed that the non drive side pedal seems a bit closer to the chainstay but the spindle(non brand) is covered fully. I consider installying a 2nd 2.5mm spacer on that side.
Regarding the BBRS500, the incomaptibility is just because there is not enough thread on it to accomodate the spacers?
In another note I have used the Tiagra 4700 front derailleur and STI shifters with 40/24 crank. Shifting is more than reasonable.