Tamiya paints- how many coats?

I'm new to Tamiya paint so I'm learning too. I think that it has more to do with the color of the surface that the paint is being applied to than who made the paint. Darker colors have a harder time covering a lighter surface than a dark one. Just tonight I did basically the same thing. I am working on an AMtec kit. They used plastic that is almost white. I preshaded the panel lines (first time ever) with Tam flat black then sprayed the XF-62. By  the time I got the plastic covered my pre-shading was totally gone. It would have been better to prime with a darker color first, like neutral gray.

I've had similar problems covering light colors with darker color using MM enamels.

I personally prefer several light coats built up to give me the base coat for a model, this is followed by several more light coats of various post shades of the base color.

Cheers

Hkshooter is correct.  It is not uncommon to have to apply a lighter or darker color to the bare plastic to maximize opacity of Tamiya.  This is true with enamels as well, in some cases..but generally a neutral gray, as mentioned is ideal.  I will usually undercoat with a complimentary color unless I'm using solid black or white.  For example, if I need to paint a scheme that calls for the underside to be Light Ghost Gray, I will undercoat with white to maximize the final color's opacity.  Other factors to concider are the kits styrene color and the finishing colors needed.  Hope this helps!