Many people, myself included, do not count erythritol. We talk a lot about your “N=1” around here - meaning “what works for you.” If erythritol does not cause an insulin release for you, you probably don’t need to count any carbs for it. Some people count half, or don’t use it at all because it causes an insulin release.
You can find posts in here on how to test yourself for reactions to different sweeteners. This WoE is pretty much all about controlling your insulin and fixing your metabolism, so it’s important to know.
If you’re really new and just getting started, I would say, use some of the stevia/erythritol mix if that’s the way you can drink your coffee, but figure out the answer to the insulin question eventually.
You can get pure stevia (I use “mommy knows best” brand on Amazon), and many of us use pure sucralose in drop or powder form (Splenda without the extra crap) because it’s very rare for anyone to have an insulin reaction to those two. There are other choices as well.