How do I change the field value of multiple nodes?
You could use the Flag module to get this to work, by configuring an appropriate (non-global) flag, which you should then use to (automagically) flag a user after such user was emailed for the first time. To actually flag such users, you should just improve the Rule you use to send those emails, by adding a rules action to flag that user with the flag you created.

Any time your existing rule is about to send another email, you can then verify if it is the very first time, or yet another time. For this verification, all you'd have to do is use a Rules Condition to check "has this user yes or no been flagged already (with the flag you created for this purpose)?".

If you're not familiar with the Flag module, checkout either of these:


The set of 8 video tutorials about the Flag module.
Flag's Community documentation.


PS: I'm assuming this is about D7 ...