The max. penalty is not much of a deterrent.

In my opinion, we always talk something or someone is ( not much of a ) something or someone.

( Not much of a ) is an idiom, it means not a good.

For example, * the maximum penalty is not much of a deterrent.

We can also write * the maximum penalty is not a good deterrent.

Or He is not much of a tennis player.

Other idioms of much :

as much / as much as something can do / this much / not much in it / much as

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PS : Idiom ( 方言、成語、習慣用語 ) ; a group of words whose meaning is different from the meanings of the individual words

"not much of a" is an idiom (成語)
and the meaning is 稱不上好,算不上好的

so, the sentence "The max. penalty is not much of a deterrent" means
最高上限罰款也稱不上是（一種）威懾