It's a bit tricky because "multiple" can act as both a noun and an adjective.
* As a noun, "multiple" means "more than one" and it is already singular.
* As an adjective, "multiple" means "many" or "several" and it doesn't have a singular form. You would need to use another adjective, like "one" or "single" to make it singular.
For example:
* Noun: "There was a multiple of choices." (meaning more than one choice)
* Adjective: "We had multiple choices." (meaning many choices)
* Adjective with singular form: "We had one choice."