The noun luggage is an uncountable noun, a form of aggregate noun, a word representing an indefinite number of elements or parts. The noun luggage has no plural form.
The possessive form of the noun luggage is luggage's.
Example: The luggage's style was very old fashioned.
The possessive form of the plural noun couples is couples'.Example: All of the couples' luggage was piled in a mixed heap.
The plural form of the plural noun passengers is passengers'.Example: All of the passengers' luggage has been stowed.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is relatives. The plural possessive is relatives'.
The plural is stations. The plural possessive is stations'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.
The plural of rose is roses. The plural possessive is roses'.