According to the London 2012 official brochure there are 302 gold medals available in this edition of the Olympic Games.
To that figure we can add 499 gold medals to be awarded in the Paralympic Games.

The exact number isn't known to the general public yet. The number always varies slightly between one Olympic year and the next, as diffferent numbers of medals are awarded within events (for example, boxing, where the number of categories can change) and sometimes a new sport is added - like synchronised swimming which has only recently been admitted as an Olympic sport.

To give a general idea, at the Beijing Olympics in 2008 there were 302 events, so the number of gold medals will have been about the same. It's not just a simple question of dividing the number of events by three, though, as in certain cases there may be more bronze medals awarded than gold ones.

You can see a full list of sports here (there are 36 in all) and if you want to find out the total number of gold medals before the event you could try the 'Ask a question' section at the bottom of the same page.