deterrence is a noun. In this structure the noun 'measures' is to be modified by an adverb/adjective. 'deterrent' is an adjective. You could use 'deterring' as well which functions like a gerund. A number of deterrent/deterring measures have to be taken immediately.

"deterrence" - it's a noun,so with "measures" must be used an adjective. The second one is right.