One of the most discussed topics in SETI is how many (if any) civilisations there are out there?
Exopolitics students and many UFO researchers will happily list off a number of different civilisations who are visiting the Earth right now!
However, astronomers and SETI researchers will go back to something called the Drake equation. This was named after Frank Drake who was asked by the US National Academy of Science, back in the 1960's, to come up with a way of estimating the possibility of extraterrestrial life. He came up with an equation with seven terms, each of which can be assigned a possible number. Multiplied together you end up with a guess as to the number of communicating civilisations in the galaxy.
An article in the April 2010 issue of "Astronomy" magazine pages 24-29 titled "How many civilisations lurk in the cosmos?" by Steve Nadis, updates our best guess. In 1961 Drake estimated there could be 10,000 communicating civilisations. Today he says "...the numbers we've been plugging in are too small." This would lead to a number higher than 10,000!
It's a fascinating article if you happen to come across a copy of the magazine.