"I drink your coffee."
Translation:Jag dricker ditt kaffe.
If you want several people's coffee [reminder: there is no royal you in Swedish, so er/ert/era all apply only to groups of people], it would have to be ert kaffe, not er nor kafe... and the implication there is that there'd be more than one coffee, so you might even need era kaffe? Only, I'm not sure that kaffe can take a plural on its own, so you might need to modify it further and go with "jag dricker era koppar kaffe", by which point you're kind of far afield of what the question is looking for.
there is no royal you in Swedish, so er/ert/era all apply only to groups of people
Ni can absolutely refer to one person, if you are being very polite. (There are threads on this topic if you look for them -- the usage of the singular ni seems to very a lot both geographically and socially, but it is explicitly stated in Svenska Akademins Ordlista (under the word du) that "ni med böjningsformer används också som sing. i tilltal till mera obekant person".)
Jag dricker ert kaffe is a perfectly good translation of I drink your coffee, and can (just as the English sentence) refer either to the coffee of one person (for example out of that person's cup) of the coffee of several people (for example nipping into the office next door and pouring yourself a mug of their coffee, instead of buying and making some in your own office).
The description is not completely correct. See this thread (read the comments too to see native speakers disagree among themselves): https://www.duolingo.com/comment/5591933
tl;dr: the traditional way of formality was not ni, it was addressing people in the 3rd person. People used titles too much so in 1865 ni was introduced. It was never quite popular and about 100 years later a new reform abolished the word again, now in favor of du. Today, some people (mostly young people in service professions) want to use ni as a formal pronoun, some don't care, many people are offended.
ni is also much more acceptable in Finland than in Sweden.