Shoplifters alone aren't enough to get this achievement. Only 27 thieves spawned in the playthrough I got it. Catch them all but keep catching other baddies as well. Catch Crew members tagging the walls and try sorting out local problems as they often result with arrest opportunities that also count toward this achievement.

Arrest mobsters trying extort money from local businesses, the boxer who caused disturbance, the drunken guy who stumbles to the car you were looking for, the mobster who tries to bully you to forget those taxis they're using to distribute narcotics.

If you see arrest option, use it. There's also a dealer standing on different corners every day. You can buy drugs from him but you can also arrest him.

But if you want to arrest him every day be careful as you'll quickly get yourself in a lot of trouble with the Crew. Do a favour for them every now and then. The same goes for the Mafia as they won't be happy with all those arrests after a while either.