It depends on the complexity of the system. A more complex system will need more stories; a simpler system will need fewer. There should be at least one story for every major feature in the system. Here's a rule of thumb: you'll probably need at least one story per programmer per month. Two would be better. So if you have ten people for six months, that's between 60 and 120 stories. If you have fewer than that, stories will probably need to be split. If you have more, that can be OK; this is just a rule of thumb. The most important thing is to get as many of the stories as soon as possible. Then estimate them as described in Story Estimation.