Put a conservative number on it.
For a starting-point, I suppose you could take the ratio of illegal aliens convicted of murder to legal residents convicted of murder, and apply it to the total (murders commited, solved and unsolved, in the U.S.).
I googled 12 murdered daily by illegal aliens and just went with the very first site pulled up; anyone who wants to look further into the methodology of how those figures were determined, etc-have at it.