The number of donors displayed at the top of your campaign page is intended to reflect the number of 'unique' donors to your campaign. With regards to how that number is counted, a donor can make multiple gifts, but if they use the exact same combination of 'email address' and 'name' they will only be counted as one (1) donor in the overall donor count as well as in any leaderboards you have enabled. However, if they change anything about how their name is typed in (ex: adding a middle initial) or change the name entirely (even if using the same email address), they will count as an additional donor. The platform was designed this way to accommodate donors giving on behalf of others as is often the case in senior gift campaigns, etc.
If you notice that one of your donors has made multiple gifts using different email addresses or combination of name and email address (and thus is being counted more than once), you can adjust the overall donor count and the leaderboards by entering negative numbers (e.g., "-1") from the "Offline gifts" tab on your Campaign Settings page.