Four charts a sheet, one question each: read every bar and add them together. It is the question on this topic that uses the whole chart rather than one or two bars of it, which is why each one gets a chart of its own - a chart has exactly one total, so there is no second total to ask about it.
That also means the questions-per-chart setting does nothing on this page, however it is set. A chart carries four to six bars, every one ending on a labelled gridline, so the addition is four to six exact values rather than a row of estimates.
Read each bar's value against the scale, write them down, and add them. With four to six bars that is a short column of addition rather than mental arithmetic.
A chart has exactly one total, so a second total question about the same chart would repeat the first. The sheet draws a fresh chart instead.
No. That setting groups several questions onto one chart, and this question type only ever supports one, so the page looks the same at every value.
Bars reach 20 at most and sit on gridlines four apart, so a total of six bars stays comfortably inside three digits.