'Ubiquity' is the noun that goes with 'ubiquitous': the fact or the state of being everywhere. You use it when you want to talk about the quality itself — 'the ubiquity of smartphones', 'the ubiquity of English in advertising'. It is uncountable, so you do not say 'a ubiquity' or 'ubiquities'; it takes 'the' most of the time, and it is almost always followed by 'of' plus the thing you mean. It is a formal word, at home in essays, journalism and business writing, and rare in conversation. Very often it carries a small judgement — a writer who mentions 'the ubiquity of plastic packaging' or 'the ubiquity of surveillance cameras' is usually about to say this is a problem, or at least something worth stopping to think about. It can also just mean plain usefulness or reach: 'the ubiquity of the network' is a selling point, not a complaint. The word began in religion, describing God as present everywhere, and that older, heavier feeling still sits under the modern everyday use.