I looked up the definition of "walk-in" as a noun in Merriam-Webster dictionary,
- a walk-in refrigerator or cold storage room
- an easy election victory
- a person who walks in without an appointment
and wonder how to spell the plural form of "walk-in" (definition 3), walks-in or walk-ins? I know a word of a similar morphological structure and its plural form is "passers-by."