What black value do you recommend. What is rich black?

Rich black in CMYK offset printing is a ink mixture of solid black over one or more of the other CMYK colors, resulting in a darker tone than black ink alone can generate in the printing process. Rich black is recommended for large, solid black areas and text over 36 points. For regular text, do not use rich black- use 100% black (C0,M0,Y0,K100). 

Recommended Rich Black for Paper stocks - C50%M50%Y40%K100%, 

White Plastic, opaque plastic, clear plastic - C20%M20%Y20%K100%,

 Silver plastic, gold plastic, silver pearl, gold pearl - K 100%, 

PVC -C30% M30% Y20% K100%