Question on publishing a series?

If you are first-time author, you query publishers and agents with ONE book. They will not buy a book from a first-time author if it only works as a series. It's fine to mention that you have others written and/or others in mind - that could be a selling point if they like your work - but they won't buy a whole series from you straight away. They'll want one book, and it must standalone - because they don't want to waste their money committing to a series when they don't know how well the first one will sell.