Capitol Police issued the crowd count estimate Tuesday. The rally was organized to coincide with a Senate Labor Committee public hearing on the issue.
Union members gathered to urge rejection of the measure. It would make it a crime for a business to negotiate a contract with unions that requires workers to pay union dues.
Opponents say that will result in lower wages for workers and allow some to get the benefits of union membership without paying for it.
Supporters of the measure say it will make Wisconsin more competitive and give workers freedom to choose whether they join a union.
The Senate was expected to debate the measure Wednesday.