Link on all the pages of one website

is it good to have on all the pages of one external website the same link ?

It wouldn't hurt. But value of every additional link on the same domain diminish greatly.

I heard that it could hurt but at best it doesnt help much. Most people when looking for a link will place it on one page of a website. This has been my experience.

if u have 10 pages, yeh, but if you have 300...you need to build a link strategy for bigger and longer term. maybe using sitemaps & sub-sitemaps.