Complex sentence: The number of electors in a borough varied widely, from a dozen or so up to 12,000 and frequently the selection of MPs was effectively controlled by one powerful patron, for example Charles Howard, 11th Duke of Norfolk controlled eleven boroughs.
Split into simpler sentences: The number of electors in a borough varied widely, from a dozen or so up to 12,000. Frequently the selection of MPs was effectively controlled by one powerful patron; for example Charles Howard, 11th Duke of Norfolk controlled eleven boroughs.