Collective nouns are words used to describe a group of objects. For example, a school of fish, a shiver of sharks. 'School' and 'shiver' are collective nouns. While 'signage' is used to represent a group of signs, we wouldn't call it a collective noun. That is because a collective noun is a different word used to describe the group.
Usually, when referring to a group of signs, we just call them 'signage'. However, that is not to say that using 'signages' is wrong. You can use 'signages' when you refer to more than one group of signs that are distinctly separate from each other.