Generally at least three or four good paragraphs. Introduce the place your RP is set, the characters, and the start of the plot, and you should be fine. Remember to be descriptive and detailed! 
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Mewtwofan259
3 or 4 paragraphs? Did you mean sentences?
_Umbreon_
Mine are mostly five or six! If you have a good Roleplay idea writing a good introduction should be easy!