There are no specific number of scenes needed to tell a story. Each medium sets its own requirements. The suggested 28 scenes is a suggestion for a "generic" story and is only intended as a place from which to start. Sitcoms are a much shorter form than novels or screenplays, so they necessarily will be explored in fewer scenes and to a much shallower depth.
We currently do not have a template for building scenes for a Sitcom. However, you might want to give the Short Story structure template a look at, as well as look at some of the scene related materials from a book we're published called, Dramatica for Screenwriters. Section Five may prove to be useful for you.
One recommendation I have for you is to determine which throughline(s) you are planning on emphasizing in the sitcom. Though you'll want all four throughlines to be represented, there isn't enough time to explore them all equally. Situation comedies traditionally tend to emphasize the Overall Story throughline. Some newer sitcoms (e.g. Will and Grace) give a lot of emphasis to the MC/IC (relationship) throughline. How you balance the four throughlines will impact the overall "feel" of the sitcom.
Another thing to think about is the effect that "comedy" has on the four domains.
- Comedy + Situation = situation comedy
- Comedy + Activity = physical comedy (e.g. slapstick)
- Comedy + Fixed Attitude = comedy of manners (i.e. a humorous clash of attitudes)
- Comedy + Manipulation = comedy of errors (e.g. gender and identity confusion, etc.)
If your sitcom is comedy across the board, then all four domains will be explored comically. How you assign your throughlines to those domains, combined with which throughlines you choose to emphasize, will effect the over type of comedy you create. Section Two of "Dramatica for Screenwriters" has some good insights into "classic" traditional genres.