There may be several answers depending on the issue.
If you are just browsing your voter list, at the bottom of the list it should say how many voters are currently viewable out of your entire voter list. You may have multiple pages of voters.
If you have any letter or character in the ‘Search All Columns’ field, it will only display applicable information.
You may accidentally be looking at Group Members, which would only display the voters of the current group you are looking at.
You may accidentally be looking at Who’s Voted, In Progress, or Not Voted which would only display the applicable information.
If you have more than 5000 voters, please view this help page.