The noun relevance can be countable or uncountable. In more general, commonly used, contexts, the plural form will also be relevance. However, in more specific contexts, the plural form can also be relevances e.g. in reference to various types of relevances or a collection of relevances.

Can I use that with plural?

The relative pronouns "that" and "which" can be either singular or plural. If the antecedent is singular, then the verb must be singular. If the antecedent is plural, then the verb must be plural.

Can the word if be plural?

The plural form of if is ifs.