This check leads to the best validation experience:
Yup.string()
.required()
.matches(/^[0-9]+$/, "Must be only digits")
.min(5, 'Must be exactly 5 digits')
.max(5, 'Must be exactly 5 digits')
output:
12f1 // Must be only digits
123 // Must be exactly 5 digits
123456 // Must be exactly 5 digits
01234 // valid
11106 // valid
Demo:
https://codesandbox.io/s/yup-y6uph