It depends on your painting technique and how many shades you use per colour. I usually use 3-5 shades per colour, so I need quite a lot (I in fact aim to get all of them). If you are only using one or two shades per colour you obviously need fewer.

How many shades per colour do you use? And how many shades per colour would you like to use?

Having lots of different paints really helps when batch painting armies, especially in "tabletop" standards.
Also helps a lot if sth gets damaged and you have to repaint it.
Trying to figure out what kind of mix did you use 5 years ago takes either lots of time or some skill. (keeping notes can also help )
Also helps if you want to make little variations in your army, for example making some soldiers skin more tanned, or getting some of the armour to look a bit more rusty etc.

Painting single models, having less paints aint that bad, since you have to mix colours for just one model so you are doing it once.
Having a great unstanding of colour theory, you can get any shade with just a limited amount of colours. Having not-so-great understanding like me, well the more the merrier.

If you can afford them, more paints are cool.
Is it gonna make your painting automatically better?
No it won't.
Is it gonna make your life a bit easier?
9/10 times yes.
Are you gonna use all that 12 shades of green that you impulsively bought?
Probably not, but meh maybe sometime.
Are you gonna use the 12 shades of gray you bought?
No way, but but it says London Grey, it's so.. so.. gray and has a british accent and im an impulse buyer and, and and..

I would agree with everything posted above. I had to laugh at Whatevier's London Grey comment. My bathroom and laundry room are London Fog. It really is such a beautiful grey, it changes hues throughout the day.

I would say that it is essential to have a good primer, because if you don't, you will put on too many layers that obscure the detail in our small minis. I would also say not to buy the GW base paints unless you are feeling plush. Just get some sand from the beach and glue that on there. Much cheaper.

I do blend paints a lot these days, however, I tend to do this either in batches or on organic models. A horse won't be the identical same colour as all the other horses in my regiment, and I think he looks better for it. I mixed up a batch of wash last night and did enough to do all the zombies at once so they would look more united.

Play with your paints a little bit. Heavens knows most of my GW pots dry out before I get to use all the paint inside anyway...

Yellow for some reason is a notoriously difficult colour to paint. I've been using GW's yellow range and I seem to be getting good results with it. Yellow also really needs a white primer undercoat. I think part of the reason that it's such a challenge to paint is that our layer paints are by their nature, designed to be semi-transparent.

So remember: For a better balrog, always prime in white!