Is it "S.Sgt.'s" or "S.Sgts."? Thanks.
Staff Sergeant is usually abbreviated SSgt without the period, in rare cases it may be abbreviated SSG but I don't believe I've ever personally seen it abbreviated S.Sgt
With that being said, in any rank system usually the rank is the one that the 's' is added to to make plural and the qualifier is left alone. For example, the plural of Gunnery Sergeant is Gunnery Sergeants but the plural of Sergeant Major is Sergeants Major (no source for this, just one of those "ooh ahh" facts I learned as a boot in the Marines).
Edit: Source for usage of "Sergeants" and Sergeants Major":
- The sergeants and sergeants major, trained now to receive instant 8 Regimental Sergeant Major respect and obedience, came home to find alien young males – Was That Really Me? by Ernest Millington
Ngram: Sergeant major - Sergeants major.
The plural of an abbreviated rank is formed by adding an "s" to the principal element in the title before the period. The examples given by the AP Stylebook are "majs. John Jones and Robert Smith"; "Maj. Gens. John Jones and Robert Smith"; and "Spcs. John Jones and Robert Smith."
Thus, the plural for "S.Sgt." would be "S.Sgts."
That said, "S.Sgt." does not appear to be the usual abbreviation for Staff Sergeant (at least in the U.S. OR U.K. military). According to several official military websites, Staff Sergeant may be abbreviated as "Staff Sgt." (U.S. Army, Marine Corps and Air Force - but see below) The AP Stylebook specifies "Staff Sgt." However, at least one source the Center for Military History Style Guide does specify "S. Sgt." (with a space).
As an aside, the military seems to prefer abbreviations that do not involve periods. The periodless form of the Staff Sergeant abbreviation is SSG (U.S. Army)or SSgt (U.S. Marine Corps and Air Force).
Don't use an apostrophe. This is a plural, not a possessive.
But do go ahead and use other answerers' advice as to the proper format of the singular form of the abbreviation.
It would be nice if you specified the context for using a plural of the contracted form. In the US, it's hard to imagine it occurring in current usage.
In the current US Army, staff sergeant (pay grade E-6) is contracted as SSG. When used in orders, it is never found in the plural. Instead, it will precede the name of each individual: "The following individuals are recommended for promotion: SSG Smith, SSG Jones, SSG Rosencrantz, SSG Guildenstern.". In an after-action report you might find something like "The two patrols were led by SSG Roe and SSG Doe.". A sign proclaiming that a facility is restricted to staff sergeants or higher would use wording like "Restricted to E-6 and above".
In civilian life, of course, nobody cares about exact military rank, particularly NCOs, and a group of staff sergeants would be referred to as "that bunch of sergeants". It's unlikely that a little girl would run to her mother and announce, "There's 3 staff sergeants at the door!"
A newspaper article describing an awards ceremony would probably not use rank contractions at all, and would say something like "Staff Sergeants John Smith, Joan Baez and Jane Fonda received the Silver Star."