When do I email someone who gave me their card?
I would say within a month. Unless this person gave you a time to contact them (ie: the middle of winter for a spring internship, next time you’re in their area, etc)

How do I follow up?
Wait a week or so after emailing them and if they haven’t replied, forward your original email to them and above it follow up, that way if they never got your email it’s already there.

How many emails is too many?
More than one follow up is too many.

When is it ok to use someone’s number from their signature?
If you’re working with them, it’s fine. If you’ve just met them, and are looking to work with/for/build a relationship with them, only when they tell you to contact them via their attached number.
DONT CALL PEOPLE YOU DONT KNOW JUST BECAUSE IT’S IN THEIR SIGNATURE!!!

Big thanks to Keaton for putting this together for us!