There is no real answer to this question because it is relative to the goals, purpose, and design of your website and its pages.
Of course, you will want something straight, to the point, and in-place. What is meant by in-place is information that fits where it is placed instead of providing information that is not relative to that page's purpose. You will also want to keep in mind to not bore your readers.
So the number of words appearing on the line will depend on the design of your website. If you have navigation links on either side of the browser, or even on both sides of the browser, obviously the number of words on a line will be less to fit inbetween. If you have nothing on the sides, the number of words will be much more.
Keeping all of this in mind, you want to make sure it is readable. It's difficult to keep track of a string of text that is written completely across the screen from one end to the other. But it is easier to read the string of text if it is only taking up part of the screen and then wraps to another line. Break it up a little bit in a way that makes sense.
Now if you are using this text as some sort of advertisement, you will want all of the above plus something short and eye-catching. About a five word string or less for this (but something that makes sense).
I'm sure this answer really creates more questions than answers but try a few tests. Make a variety of pages with various layouts of a varying number of words in a line and you will see what I'm getting at here.