How many 1943 wheat pennies are still out in circulation?

No agency keeps track of the number of bills or coins with a specific date that are in circulation. It would be impossible to know how many are lost, damaged, squirreled away in drawers, etc. so that's really an unanswerable question.

In any case there was an effort many years ago to recover and melt many 1943 cents because they were corroding. They're also sufficiently distinctive that any that did remain in change were probably taken out as curiOSities or by collectors, so the actual number remaining in change is probably vanishingly small.