The National Labor Relations Board provides data on election petitions. They provide a record for each opened election related case. Which means that if a case is closed and then re-opened a new record is created. This means that a single union campaign can be in the NLRB data multiple times.
Why is thing missing from the data?
The NLRB data is spotty at times and so there is occasionally missing information.
What sorts of elections are included in the data?
The NLRB only governs elections at private organizations and so excludes public employee elections. In addition, the National Mediation Board governs elections for railroads and airlines so these are also excluded. For simplicity, this website focuses solely on elections to form new unions (and also excludes the rare case where multiple competing unions are involved).
Union elections happen after employees petition the NLRB to hold an election. A minimum of 30% of the potential union have to sign the petition. At this point employers often contest who should be in the union. Once the set of employees is decided (called the bargaining unit) an election date is set. Employers often run aggressive anti-union campaigns at this point as well.
What other types of elections happens?
Along with elections to form unions, the NLRB governs elections to disband unions.
How is the election decided?
It is a simple majority election, and like other elections, it is a majority of those who vote.