A rough numerical approximation: I'd say forty, but that's a ballpark figure(1960s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with ballpark figure
An acceptable, roughly accurate approximation, as in I know you can't tell me the exact cost; just give me a ballpark figure. This term alludes to a baseball field, which is always an enclosed space. The expression is basically an extension of the somewhat earlier
in the ballpark, meaning within a reasonable range, and
out of the ballpark, beyond a reasonable range.
[ ; late 1960s