Copic markers are one of the most well known and recommended brand for graphic markers by artists. However, it does cost a fair bit of money, as well as the other brands of graphic alcohol-based markers. If you're not of the rich side, it's most likely that you buy one marker after the other, and that's a hard thing to do - choosing which colours to buy first.
How I would've approached this situation is that I'd buy two shades of each colour (red, yellow, blue, skin, grey ect). I'd emphasise getting a dark grey colour such as C-7 since it's useful when it comes to shading. Then I'd buy a black and a colorless blender marker. Overall I'd most likely have 20 Copic Markers. From there, you can build up with by buying more shades of the colours you mostly use.
The image below is a comparison of how I coloured with only 5 Copic Markers and the other one with 9. The image on the left is where I used the grey colour to shade and I only had two shades of brown for the skin colour. The image on the right, I have 5 shades of skin colour and didn't use a grey since I had a dark brown to sue for the shadows.