Can I store eSIM on multiple devices even if it's inactive?

eSIMs are only stored on the ONE “assigned” device.

You can’t make “offline backups” of them.

You can however move an eSIM to any other compatible device over which you exercise control with the carrier’s assistance.

An iPhone can hold quite a few eSIMs (Apple cites a capacity of “At least 8”)

If you need a replacement eSIM on a device it can ONLY be provisioned by your carrier.

Note that while performing a “Quick Transfer” of eSIMs between phones “seems” like you’re doing it …

… it’s REALLY the carrier performing all the reprovisioning steps necessary to move the eSIM onto the “other” phone in the background.

Note that some carriers might optionally provide a means for authenticated subscribers to obtain “self-service” eSIM provisioning thru their online subscriber account management system.

While certainly convenient for a subscriber needing to replace a lost, stolen, or damaged phone; it also provides a potential attack-vector for perpetrating SIM-Swap Fraud.

Any carrier offering this capability would need to also provide appropriately strong, fraud-resistant mechanisms to counter this threat.