If a spell targets a creature you control with a +1/+1 counter on it more than once, Swarm Shambler’s ability triggers only once.

An ability that triggers when a creature becomes the target of a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

You create just one Insect token, no matter how many +1/+1 counters the target creature has on it.

If a creature without a +1/+1 counter on it becomes the target of a spell an opponent controls, putting a +1/+1 counter on it after that won’t cause Swarm Shambler’s middle ability to trigger.