We'll find out for definite by the summer break as Mercedes said they will finalise their line up for next season by the summer.
I think Lewis will extend for 2 years and Bottas will be extended for 1 more season possibly his last after this with Mercedes.
Two reasons for this, 1) Keeping Lewis happy. Bottas is a known quantity for Lewis, Russell isnt.
2) More importantly, the biggest changes in F1 for 30 years. Ground Effect areo returns after being banned in 1982. 18 inch tyres, no tyre blankets, torsion bars, heave springs and inerters banned. Gearboxs frozen from development till 2025. Engines frozen from development till 2026. One set of pads and discs per race. Cars in park femme from start of FP3. Sliding scale for aero wind tunnel time and CAD time depending on how well you do in the Constructors championship the season just ending. Theres loads more stuff, without doubt the biggest changes for 30 years to F1.
The cars will be heavier and slower by up to 4-6 seconds compared to this season. All of the above mean its essential that you have known reference points for your new cars. Hence i think Bottas will stay as he is a known reference for the team, experienced and Russell isnt. Otherwise if it wasn't for the huge rule changes id expect to see Russell in the other seat.
Regardless, someons is going to nail / come up with a unique idea ala brawn double diffuser and no-one knows who that will be. You can't really rule any team out. Therefore stability in your driver lineup will be key, hence i expect to see pretty much the same lineup for all the teams next season. Its why Alonso came back and went to Alpine and Seb to Aston Martin because you might just get luckily for one season and end up a WC like Button did with Brawn.
Long winded explanation of why i think Bottas will stay next season. Btw personally i don't rate him as a driver.