Do the effects of Shard of undoing stack if I have multiple in play?

Yes, the effects of Shard of Undoing stack. If you have multiple Shard of Undoing cards in play, each one will trigger independently when the condition is met.

According to RiftJudge FAQ #10219, if you have multiple copies of Shard of Undoing and a friendly unit dies, each copy will trigger its ability. If you satisfy the "first time" condition (for example, by having multiple units die simultaneously, or simply by the first unit dying in your Beginning Phase), each Shard of Undoing will place its own individual "each opponent must kill one of their units" ability onto the chain.

When these abilities resolve, your opponent will be required to kill a number of units equal to the number of Shard of Undoing cards you control, assuming they have enough valid units to sacrifice for each trigger.