A YAML sequence is an array. So this is the right way to express it:
key: - string1 - string2 - string3 - string4 - string5 - string6
That’s identical in meaning to:
key: ['string1', 'string2', 'string3', 'string4', 'string5', 'string6']
It’s also legal to split a single-line array over several lines:
key: ['string1', 'string2', 'string3', 'string4', 'string5', 'string6']
and even have multi-line strings in single-line arrays:
key: ['string1', 'long string', 'string3', 'string4', 'string5', 'string6']