What is the maximum value for a drop down list?

You mean the number of choices? I do not know of a concrete limit. But performance and UX will degrade when that number becomes 3 digit or more.

There's no one-size-fits-all answer for the exact number where performance dips in dropdowns.
Large datasets in dropdowns can slow things down. I would suggest, For a faster search, Users simply type 3 or more characters to narrow the list and find what they need quickly. This keeps things speedy and user-friendly.