Merriam-Webster gives either "banjo" or "banjoes" as correct for the plural of "banjo," although, as a general rule, the alternative with the fewest letters is preferable.

Here's my take on it:

Singular—

banjo

Plural—

banji

Back to singular again—

banjum

Get it?
Got it!
Good!

Don Firth
(Another slow day at the skunk works)