The number associated with an ND filter is actually the denominator (bottom) of a fraction.

So an ND2 filter should be thought of as 1/2 the amount of light being allowed through the filter. For example, setting the lens at f/2.8, and using an ND2 filter would make that an f/4 situation for a total of 1 stop difference.

ND4 filter is allowing 1/4 the light (which is half of ND2) thus a 2 stop difference.

Continuing, ND8 is 1/8 and three stops and, although I've never seen them, an ND16 is half as much light as ND8 so would be four stops less light.

The decimal numbers you mention (0.6, 0.9) are another system of quantifying the density of the ND filter. These numbers are the log (base 10) of the factor by which the light is reduced. (This is sometimes called the absorbance). So for example a 1 stop filter reduces the amount of light by a factor of 2, and log(2) = 0.3 so a 1 stop ND filter is ND0.3 in this system. Similarly 2 stops is 0.6 and 3 stops is 0.9. The combined effect of multiple filters is obtained by adding up the numbers. For example a 1 stop, 2 stop and 3 stop filter combined (6 stops in total) would be 0.3 + 0.6 + 0.9 = log(2^6) = log(64) = log(2) + log(4) + log(8) = ND1.8.

I would highly suggest the best quality GLASS filters you can afford. Cheaper (especially plastic) filters will tend to add nasty color effects. Although technically color casts can be corrected in post, cheap filters also can also reduce the quality of light meaning things like more chromatic aberation.

Lastly, don't worry about getting the highest ND number, I carry two filters around and stack them together, when needed, for combined affect. Which is more reason why quality filters matter as stacking simply magnifies imperfections too!