When is enough and when is too much?

I struggle with knowing when someone expects a reply to an email and when they don't. And how many replies. Like, they send me a message, I send them a reply, then they reply to my reply thanking me for my reply. Do I reply to their reply, so they know I received their thank you? Cause to me, that seems stalkerish. Like, they may think I want to be friends or something, when I don't. Or they might think I don't think they are busy and have time to start a conversation of replies.

*On a related note, I may either need more or less caffiene*