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There isn't any magic number.  But, the more light you can produce, the better.  I just purchased some Alien Bees.  They're a good value and very durable.

www.alienbees.com

If you can afford it (I couldn't), get a softbox and other light modifies and diffusers.

if you go with alien bees, the 400w/s units should supply enough power for most things including portraits and the like.  Especially if you have 2 or three flash units.

I can also second the vote that a lot of people seem to really like the alien bees and they are a very reasonable price.

oh!

Dang!

I use Elinchrom Style Fx-400 series light (220v). They cost around 1000€ with stands, umbrellas, softbox and a hard case. There is also a cheaper light units made by Elinchroms (d-lite2 and d-lite-4) and the price is for these items are starting from ~600euros....

I'm happy with style-fx series light. Nothing more is needed at my skill level

br, Mikko P.

What I was taught for studio portrait lighting was, at a minimum, to have one 650W main light and two 350W, fill and back lights.  In some cases you'll need more, primarily for lighting whatever backdrop you have, but these, along with various reflectors, should get you by for most circumstances.  These would be for a single person portrait.  For full body or group shots, you'll probably need either more lights or higher wattage fixtures.  Like professorgb says, the more light, the better.  At least until your model begins to develop burns or cancer from exposure.

Also, these are photography lights, not cheap tungsten or halogen lights that use up their wattage in heat, rather than lumens.

This variance is true of most lighting, although maybe not as much as 1/3 stop.  I think that the 1/3-stop variance you're referring to occurs when the output power is cut significantly.  At those low outputs, all lights vary a lot more, both in power output and color temperature.