Missouri has similar conceal/carry laws, and one of the issues around Busch Stadium (which, unlike MKE, is downtown) is people breaking into cars looking for guns to steal. My hypothesis is that people are looking for people going back to their cars because they forgot to leave it there and then targeting those cars. Considering that there are 81 home games, that I'd be surprised if 10% of fans even own guns much less handguns (rifles/hunting guns much more likely), the large number of families/children that go to games, and the large number of season ticket holders (who know not to bring guns with them), I'll guess it's not many and go with a dozen a year.