Unless you are [i]really[/i] lucky, you will always have a few dropouts, be it during IntCk phase, OoC only phase or IC phase. When we run RPs, we consistently aim for getting a certain amount of players above our target player/character count, simply because we pretty much know some won't stick around.

Some tips:
Don't rush-launch RPs. Take your time.
Be strict during CS process - Saves you work later and removes players who don't fit well.
Make sure your players are engaged with each other. Force them to connect their characters so that they'll work as a group, rather than individuals. That makes them more likely to stay.