How to Set a Limit to Number of Voicemails Recorded

That would be a great question to ask your carrier as Voicemail is a carrier provided feature. To my knowledge, there is no way to do that. Certainly there is no setting on iPhone for this as again, voicemail is a carrier feature.

Wait until you get a full notification, then delete a couple of emails. I believe "full" is based on the total storage used, not the number of messages, though I could be wrong.