How common is a hydrosalpinx after a hysterectomy?

Not uncommon. For a hydrosalpinx ( fluid in the tube) to develop the tube must be blocked at two places. A hysterectomy ligates or ties off the tube at the junction with the uterus. If that tube is blocked at another point along its length( which may not be known) then fluid may collect in between the two blockages forming a hydrosalpinx. Most of the time it causes no problems.