Answer:
Let's say 6 statements are lies.
A's 1st statement and
C's 1st statement negate each other. One of them is a lie.
C's and D's negate each other. One of them is lying. Therefore, there are 4 additional false statements.
Assume A is guilty. If so, A's second statement, B's second statement, and D's first statement are the additional lies.
Assume D is guilty. If so, A's second statement, B's first statement, and D's third statement are lies. This also only makes five false statements. D did not do it.
Assume
C did it. If so, A's second statement, D's first and third statements are false. This again, makes only five false statements.
After ruling out suspects A,
C and D, B is the culprit. B's third statement,
C's second statement, and D's first and third statements are the additional false statements. This adds up to six.