A dumb question that needs answering

So I've been thinking about this for a while and I'm still not sure what the situation is. In Destiny, the game is supposed to be a shared open world. What I've been thinking is how many people can be in your version of Destiny at one time. Like if I'm hanging out at the city will there be hundreds of thousands of guardians all running around in this shared world, or will it be more like just your fireteam/friends that you can see. We know about public events in game where other fireteams collide with your own fireteam to help you take down a boss. So does that mean only you and two of your friends can truly go out and explore together at any one time? Maybe numbers of players will be restricted by certain areas. So what I'm really asking is how many people will you be seeing while on your journey in Destiny. Will you see random guardians sitting down somewhere or can you only join other players in game through public events? I could have missed something but some clarification would be nice!