Is there a way to see the number of people currently logged in? If not is this something we should add?
This information is not displayed, nor should it be. The mere fact that phpBB shows it is a reason enough. Too many people mistake SE sites for forums; the interface should not reinforce this misconception.
When you write a question or an answer, assume that nobody is online now, and will not be until you step away from your computer. Make sure that it is clear and unambiguous without any extra commentary from you.
Math.SE is not a social networking site, so there is no need for the "typical" user to need to see this information. I do think it would be useful for moderators/site leaders to know, so we can get a feel for what hours may need the most moderation, etc.
However, there is one place on Stack Exchange that supports a "who is online" feature: the chatroom. That section of the site is a border-line social network, but is also good for pointed questions to specific users. A limitation, though, is that it only shows people who are in the chatroom.
tl;dr: "Who's online" isn't needed for the functioning of the main site; but, if you really want it, there's a similar (albeit limited) feature in the chatroom.