It's a combination of how many you have and how long you've had them (under the current system that is - the new wallet takes a different approach) so there's not really a numerical minimum, per say. The combination of how many coins you have and how long since they were last used to stake creates your network "weight" which is then compared with the weight of others on the network, and basically you end up queuing up for your chance to stake and get a reward. Also keep in mind when you stake, a portion of your coins are used for the purpose each time, not all of them at once.