The plural form of introduction is introductions.

Does introduction have a plural form?

The plural form of introduction; more than one (kind of) introduction.

Is beginnings singular or plural?

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