As reported by Counterpoint, six electors can call a Town or Parish meeting. However I anticipate issues that could be problematic.
By default, the chair of the meeting is the Chair of the Town or Parish Council. I suspect they can decide what is on the agenda, and could potentially try to prevent what the electors seek to have raised being discussed. If the Chair of the PC/TC does not attend, I think one of the electors could chair it.
Normally, PC's/TC's hold a scheduled Town or Parish meeting I the electors try to convene a Town or Parish meeting which is in addition to the scheduled one, an awkward chair could argue that they should have raised the issue(s) at the scheduled one, and as they haven't could try to prevent what the electors seek to have raised being discussed.
One way that electors can impose their views on a Town or Parish Council is through a Parish Poll.