Depends on the nature of the cronjobs you're scheduling. If your cronjobs cannot run in parallel on the same node (or, more likely, you cannot trust that they can safely run in parallel on the same node, because somebody else wrote the job and didn't need your review or approval before deploying to the scheduler), then you need to restrict each Jenkins node to a single executor, and you cannot run more cronjobs in parallel than you have Jenkins nodes, or else those cronjobs will be delayed. Because Kubernetes enforces the use of containers, multiple jobs can be run on each Kubernetes node with no issues (by design).

Remember - if there's a one in a million chance of a collision, it'll happen by next Tuesday.