What is the maximum number of definitions allowed assigned to one inline C var

I'm not quite sure what you intend? You want to just define an array? If so, you just define that array in your Inline C code:

var c = E.compiledC(`
// double lookup(int)

double lut[8] = {
 1,2,3,4,5,6,7,8
};
double lookup(unsigned int x)
{
  return lut[x&7]; 
}
`);

I don't believe there are limitations on the number of items you can have in Inline C, but each one uses a JsVar to store a reference to the function, so you'll quickly use up your device's memory.