Re: How many different ink colors do you use?
I have great book by Michael Wilcox, 'Blue and Yellow Don't Make Green'. If you can get it, it gives some great advice on colour mixing.
In short, you need two of each of the primaries and black and white, or 8 in total. The two of each should be a red with yellow undertone (to mix oranges), red with blue undertone (to mix purples), blue with yellow ut (for greens) and blue with red ut (purples), yellow with blue ut (greens) and yellow with red ut (oranges).
The undertones are very subtle but when you mix colours they are crucial to get the right shade. In theory there are no pure primary colour inks and if you mix inks with the wrong undertones it gives greyish colours.
Personally I use black, white, yellow ochre (red undertone), prussian blue (green undertone), burnt sienna, and umber - but my palette is limited, and I have a few other inks I haven't even opened. Get the book, it explains it very clearly and you will never be stuck again choosing inks.