When resolving the effect of Heartless sacrifice, are the effects of each individual exploding mummy resolved on all figures in contact (including corner to corner contact), which would result in figures being subject to multiple attacks or is each target figure only subject to one attack no matter how many mummies are in contact?

For example, a Beloved in melee with a unit of horde mummies that explode could be subject to between 3 and 4 attacks depending on exactly how it was in contact with the unit.

A formation that is wrapped by mummies could have the figures on the corner of the formation subject to up to 5 attacks, while the front rank figures subject to up to 3 attacks.

I'd note that the second rank of the mummy unit would also be subject to the attack. It really is a sacrifice...

Only a single attack for each model in base-to-base contact with the mummies affected by the spell - the mummies are treated as a single unit when calculating the attacks.