I started with 4 shellac colours, lol.
I went with a pink (sure it was gotcha), French white and 'pink and.... silver chrome because it was really cool! Lol.
I wouldn't recommend the silver if I were starting again, but I got iced cappuccino (sadly no longer available) as I could use it on its own, over the French and also over the pink for another colour. So I had 6 colours for my four bottles.
I would recommend a light pink, a brighter pink, red (hollywood), a dark (dark lava for me), a purple and field fox as an absolute minimum. Also a French pink and white - but I think I've only done one French in the past year.
Then, as trinity and CFBS say, but more colours in your good months.