The singular possessive would be waitress’. The plural possessive would be waitresses’.
Usually the possessive form is indicated by an apostrophe followed by an s . However, it can be quite confusing when the noun itself ends in -s. The plural possessive of such is directly an apostrophe without an additional s. As in teachers’ (of the teachers). The singular possessive can be either of the aforementioned. As in Jesus’ or Jesus’s . Though most people tend to use the former one (Hence I wrote only the former version in the main answer.)
