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

What is the plural form of buses?

The plural of the noun bus is buses. You might see the plural busses, but that form is so rare that it seems like an error to many people. You might also see the verbs bussed and bussing, both of which are rare and also come across as an error to many people.

What's the plural of insurance?

insurance. Plural. insurances. The plural form of insurance; more than one (kind of) insurance.