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']