The standard two-camera setup for a seminar/lecture is a Mid to Medium Close shot & a Wide Shot on the presenter. If there are slides as part of their presentation, then the Wide Shot camera gets those as well, (- these can also be inserted during the edit but it is time-consuming). If there is to be audience participation then a third camera behind the presenter onto the audience is recommended, although for those on a tight budget the 2nd camera could shoot this, leaving only one from the front. Even without audience participation, for those with enough budget, the third camera adds a third perspective, although I must admit it is used sparingly in the edit.
For those on a very tight budget, a single camera can work, which I have shown in the following clips. The first presenter does not use any slides; the second does, but were inserted during post; the third, which is extremely short, does not use slides but chose to have ‘cards’ inserted during post’; and during the fourth clip, the camera moves from a slide presented during the talk, down to the presenter.