If the Russians incorporate the Gsh-23 as a coaxial gun (like on the BMP-3) on a 57mm armed T-15, the amount of 23 X 115 rounds that could be accomodated in the turret would be contingent on the size of the turret that the T-15 Armata ends up with.
Being and IFV it will still need guided ATGMs, so Kornet or Ataka or something like that would still be needed.
Personally I would fill the area below the turret ring with 57mm rounds... 150-250 would be ideal IMHO, but that will likely fill the area completely. Therefore I would make the 23mm cannon coaxial and have an ammo drum behind the rear of the turret with 400-800 rounds... the 23mm rounds being much smaller than 30mm BMP rounds and would take up rather less room.
The risk of them cooking off if hit by enemy fire would be low so external storage or secondary ammo should be OK.
If the Russians incorporate the Gsh-23 as a coaxial gun (like on the BMP-3) on a 57mm armed T-15, the amount of 23 X 115 rounds that could be accomodated in the turret would be contingent on the size of the turret that the T-15 Armata ends up with.
The rate of fire of the GSh-23 is enormous... there would be no advantage to having two unless the aim is over kill.
The aviation gunpod using the same cannon weighs about 220kg, where the gun and 250 rounds of ammo weighs about 145kg.
The 6 barrel gatling gunpod with 500 rounds weighs 260kg with gun and ammo, so the ammo is compact and light, as are the guns that fire them.
For some reason Russians don't like putting secondary guns on their AFVs that aren't larger than rifle calibre MGs while other countries have experimented more with them, while for example the french have been putting 12.7mm browning coaxials on their leclercs and the czechs made the T-72M2 Moderna which had a 2A42 strapped on to it.
Except for BMP-3 with 100mm rifled gun and 30mm cannon, or the BMP-2 upgrades with 30mm cannon and 30mm grenade launchers mounted to the rear of the turrets... and of course ATGMs.
In combat a machine gun can be rather useful and can carry a rather larger amount of ready to fire ammo than a grenade launcher or heavier weapon.
The T-10M has a 14.5mm coaxial HMG and a roof mounted HMG that replaced the 12.7mm weapons used in both positions on the T-10 tank.
The reality is that many targets appear within about 1km where a rifle calibre machine gun is still very useful.
Most Soviet/Russian APCs have HMG calibre main guns, so there is no shortage. The 14.5mm HMGs are very similar to western 20mm cannons with lighter HE rounds but much better armour penetration performance.