I have started a blog on my site using K2 categories. However the urls of posts are strange. K2 add item/number in urls. Like www.domain.com/blog/item/14-importance-of-breast-feeding
How to remove item/14- etc because it is not good for seo.

The /item/id won't affect your SEO. I think the ID part is requested by Google.

If you still want to remove them you can use either K2's advanced SEF or an SEO/SEF extension.

More on SEF can be found here: getk2.org/blog/1159-k2-v264-release-introduces-new-cool-sef-url-options