Is there anything else to save up Sinnoh Stones for?

Ralts CD for Gallade. Rhyhorn also stands a good chance at getting a CD because it has a signature move Rock Wrecker. There's also the remaining unreleased evos that will probably use the stones as well.

Yanmega is among the top for bug type DPS, Tangrowth is solid and Magnezone will be among the best electric types.
Also you may want to have one of each new Eevolutions

I'd say save them, theres not much of a need for any of the evolutions right now. Some of the evolutions are good, but arent expecially necessary at the moment.

Ralts may (?) get a CD in the future, Rhyperiors likely to get Rock Wrecker, its former signature move. Magnazone will be a great electric attacker, its steel typing will it more TDO as well as spectacular DPS, more TDO due to resistances other electric types lack.

I dont think Gible will need stones to evolve, thats limited to cross gen evolutions

The two eevee evolutions when they come out.

I've evolved 44 times using Sinnoh Stones, now I have 21 Sinnoh Stones left.

2 dusknoir
3 Porygon Z
12 Mamoswine
1 Gliscor
3 Yanmega
1 Togekiss
1 Magmortar
1 Electivire
1 Tangrowth
5 Weavile
4 Honchkrow
4 Mismagius
4 Roserade
2 Rhyperior

If you have no immediate use for the evolutions, then din't bother.

Leave your options open, and choose only when you need to.

You already have plenty of Mamoswines and none of the other Sinnoh evolutions are essential, so I think you can pretty safely focus on filling your pokedex at the moment.
If you want to play it safe, save 3-4 in case of something unexpected.

There are still 4 unreleased crossgens that will most likely use stones, but none of them are essential either. Although if you're into great league PvP, you'll want a Probopass right away.

Save them or at the very least don't evolve your best ones. Dex filling can wait but if you're impatient just evolve some shitty ones. Out of all the current ones, the only ones that have (real) viability are Mamoswine, Electivire, and Roserade. By that I mean that they are probably already the best they can be while the rest could have some major move improvements.