How to know the exact amount of hops needed by most of beer styles?
Is there any table or something that guides the amounts or something?
The BJCP Guidelines give recommended hopping levels for most styles. The amount of bitterness is expressed as International Bittering Units (IBU). The IBU rating for a beer is a function of: quantity of hops, bitterness of hops (alpha acid percentage), boil time, and gravity and volume of wort. There are lots of good IBU calculators on the web. Google for yourself, but here's one and here's another. (I'm not vouching for the utility or accuracy of either of these tools; personally I use BeerSmith, a desktop program.)
If you want to understand the math behind the calculations, Palmer's How to Brew has a good section on hop utilization.
As well as bitterness, which is addressed by Tobias' answer, hops also provide flavor and aroma. As with bitterness, the BJCP guidelines also lists typical levels of hop flavor and aroma, from none/slight to full/strong for the various styles of beer.
As a starting point for a 5 gallon batch, 1/2 oz produces a small amount of flavor or aroma, 1 oz produces a moderate amount and 1.5-2 oz produces a full flavor or aroma.
A good way to become familiar with the quantities of hops used and how they influence the final beer is to brew some well-known recipes, and look at the hop schedule. The bittering hops are boiled in the region of 30-90 minutes, while flavor hops in the region of 45-10 minutes and aroma hops from 15-0 minutes. These aren't definitive - just an estimate of the top of my head that I think reflects traditional brewing practice, and you'll notice that the ranges overlap. This is because aroma, flavor and bitterness enter or leave the beer at different rates:
It's the length of time the hops are boiled for that determines the quantities of bitterness / flavor / aroma, and you'll see these different boil times in the recipes on the net.