question

We've seen free-to-play games with a few hundred MAU, and others with millions. Predicting where your game will fall depends upon a lot of factors, including: Is it a known IP with a following? Are any of the designers/developers well-known? What are you doing to promote the title (ads, presence at E3/PAX/etc.)? Do you have any feedback on whether you'll get feature placement on the markets where your game will be available?

Some things you can do are:

• Have a soft-launch period in a small market, so that you can tune the game and check your assumptions on what players like about it (adjusting as needed). Don't just listen to forum feedback, either - usually, only about 10% of your users are going to be vocal. So get the data for their actual play and analyze that (how often do they come back, what do they do most/least, etc.). That way, when you go to global launch, you'll have a better shot at getting viral spread through word-of-mouth.
• Work with your contact at the companies that own the marketplaces where your game will be available. Just knowing people doesn't guarantee placement, but not talking to anyone makes it hard to get noticed by them, so that they can evaluate the title. A lot of titles ship each year, and they can't look at all of them, so you need to pitch yourself well to those that are doing the evaluation.
• Find out what communities enjoy the kind of game you're making. Do they have favorite hangouts online? Favorite YouTube reviewers/personalities? Finding good ways to get your game noticed there - not just throwing up messages or spamming them with it, but piquing their interest - can be very beneficial.
• Plan out your marketing expenses early, and make sure this is done in conjunction with your planning for your updates. So in addition to where your promotions will appear, it's key to know how you'll promote each update, since each is a new opportunity to draw in new players and to re-engage with your existing players.

There's certainly much more that could be said here - there are whole books available on this topic. So everyone else should feel free to chime in with their thoughts!