You could work it backwards.
Calculate how much capacitance you could drive to account for say 10
mA of drive at the desired SPI frequency, divide that by the maximum
number of  slaves that you would ever want to use, and see if the
capacitance per unit seems much higher than you can reasonably expect
it to be. If not then more work is needed.

Put another way - at the clocking speed you are working at, with the
available current, how much capacitance can you have to allow a rise
time of say 3 to 5 times faster than a bit time. The answer MAY be
"distressingly little".

R
--=20
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