AWA explains there there is no limit to the number of advertisers per search phrase.
[Link to quote
Even more info: any number of advertiser can advertise on the same keyword - and the ads are shown 8 per page.
For example, if 20 advertisers are using the keyword "slightly bent widget", ads 1 through 8 will be on the first page, 9 through 16 on the second page, and 17 through 20 on the third page.
Also, it is quite possible that the number of ads will not always every time you search. This is because some advertisers chose a daily budget lower than that required for their ad to show every time. As a result, there will be portions of the day in which their ads won't appear.