How many people to target with FB ads?

That's far too open ended to give you an answer. You've given no information on the kind of niche you are working in.

Best answer I can give you is to find what area you have the most fans in as well as targeting their demographics (age, sex) and see what happens. If there are certain keywords for your fanpage, go after them.

Generally, the more people you target the longer a campaign can last, but the less targeted it will be meaning you might spend a bit more per conversion as you'll have a lower CTR. The tighter your targeting the more likely you'll have a better CTR if you can write an ad specifically for those targets. (Of course if your page is about cats and you're targeting people that love surfing in Hawaii, that could be counter productive).

A good range of people will be 10k - 100k, but I have campaigns that work well targeting 1k to 5m.

Let me give you a rule of thumb:

Small demographic : Low campaign reach, Higher CTR in the start, High frequency, Short Life Ads, Higher Conversion

Big Demo: High campaign reach, Lower CTR, Low Frequency, Long life ads, lower conversion.

Small demographic is hard to optimize and manage. Your ads will die very shortly due to higher frequency and ad-blindness & ad-fatigue just kicks in very shortly. You need to seed your campaign with fresh ads on daily and even hourly basis.

How large is the fan base to start with?

You could advertise to the whole group, and see what ages and sex converts.

JCSS and Justin just posted some great feedback as well. But you really need to give up some info, good luck.

Thanks for the help so far guys, I know my question is quite open ended and hard to answer! I'll try and give a few more specifics, I'm targeting nfl fans and fans of a certain nfl team in the us which currently amounts to 30 million people, I'm now starting to think this may be way too much! Although I'm optimizing for likes rather than manually bidding so my reach is quite low at the moment but slowly going up, my ctr is quite good at 2% but I'm, only getting around 70 likes per day for $5 spent so my cost per click isn't really low enough at the moment. I've ran a couple of ads before where I was spending $20 a day and getting 2000 likes per day with a similar ad campaign targeted towards nfl fans so I know it can be better, I think I was targeting around 8 million with those though so like I said I think I need to target less people, I was just under the impression that the more people you target the lower the ad price is, also is there a reason my reach is only 10,000 at the moment despite the fact that when I set the ad up I set the target 30 million? I seem to remember before setting up ads and getting around about the figure I set it to target even when optimizing for likes

Unfortunately there is no way to properly answer this. It's best if you just test it based off of your own campaign and find out for sure instead of guessing