[Imp]

Okay some errors here regarding how they work, from the guide & confirmed by playing

All Ammo units resupply to adjacent hexes & the hex they are in.
All 7 hexes are supplied at the same rate no advantage to being in its hex.

Ammo Dumps & Bunkers also supply to the next ring of hexes at a slower rate or put another way supply range is 2 hexes not just the adjacent hex.

Cannisters/Containers can only supply weapons up to warhead size 6.
Look at unit specs small arms mortars & light artillery as a guide.
These also supply at a lower rate I think never bothered to find out.
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Resupply rate is reduced or negated if the unit being supplied fires or due to suppression. Larger the warhead the lower the supply rate.

They can supply multiple units at a time but top up the first unit till its full before starting on the next.
Units are supplied in purchase order B0>B1>B2 etc and topped up smallest armament to largest (warhead) before moving to next unit.

Example tank will fill its MGs then main armament. Once full it will start filling the next unit. Artillery normally has only fired main armament so tops off that & then starts filling next.

They provide a big advantage the AI does not have as limited ammo units can be topped up & returned to battle.

Hope that answers your question, as a note far safer to move the unit to the ammo supplier as a rule as part of relocating arty to prevent counter battery fire.