This article says that recall petitions have been launched against sixteen Wisconsin State Senators, eight Republicans and eight Democrats. The recall petition needs the signatures of 25% of the number of people who voted in November 2010 within that district. The article says that this number ranges from 11,817 signatures in the district which had the fewest voters in 2010, to 20,973 in the district that had the most.
The November 2010 vote in Wisconsin for Governor was 2,158,974. It is estimated that Wisconsin has 4,200,000 people eligible to register to vote. Wisconsin has 33 State Senate districts, so the average district has 127,273 persons who are registered, or who could register. Thanks to Politico for the link.