In Kanjinchō the guards speak of "a great crowd of yamabushi" — but there are only five. Why?

This is a sharp question, and it touches the root of kabuki's idiom. To answer directly: on the kabuki stage, number is not a count but a convention.

The same convention is used everywhere else. When a few hanayoten come and go in a fight scene, they are not a few men but an army. Stretch one wave-cloth across the front of the stage and it is the open sea. A matsubame stage holds nothing but a board painted with a pine, and can be a mountain path or a barrier gate. Kabuki is not a theatre in which what is visible is to be counted.

In Kanjinchō, Benkei and the four retainers on stage are the visible part of Yoshitsune's whole party. When the guard says "a crowd," that becomes the fact within the play's world: the words carry more authority than the count on stage. One further point of background: Kanjinchō is a matsubame piece transposed from the noh Ataka. Entrusting much to words and to the audience's imagination on a bare stage is an aesthetic inherited from noh, and its premises differ from those of realistic theatre altogether. Note that the exact wording can vary with the performing text.