How can I consider that how much percentage of traffic coming from yahoo. And how can I increase the percentage of traffic from yahoo?

Percentage of traffic from Yahoo is based on industries - some would be getting little and some none.

Build more backlinks from directories and sites that lists when you do a search in Yahoo.

Best,

To see your own percentage look at your analytics - to increase look at the keywords from Yahoo - again from analytics - and see what position they are in - I use seobook's rank checker to see where I am ranked for the big three engines and keep a weekly list of their positions - I would work on the ones that are driving traffic first but work them all over all

Percentage of traffic comes from Yahoo is different as time passes. But, You can know it using analytics tool for your site.

as far as related traffic the percentages are 66.2 Google, Yahoo 16.4 and Bing 11.8 - so you can get a little idea of general percentages as they apply to you