This should obviously be spoken, rather than written:
If a feller met a feller in a field of fitches, could a feller tell a feller where a feller itches? How many fs in that?
Btw 'feller' is the way that 'fellow' is pronounced in this rhyme. Fitch is, apparently, another word for vetch. It would not usually be used in the plural form, as you would say, "a field of fitch" rather than "a field of fitches."
Anybody else heard this rhyme? Similar to "Constantinople is a very long word; if you can't spell it, it's really absurd."
All correct. But if you did try to count the number of fs in that rhyme, without having it written down in front of you, a lot of people would come up one short, as they would miss the f in 'of'.