This will be mostly dependent on the goals of the community.
The one metric that almost always applies is Active Members, meaning members that have engaged (posted, commented, created a blog/poll etc.; but explicitly NOT only logins or likes). If that number rises, you have more people engaging with your community. It is the single best proxy I know for a healthy community.
That said, it is easy to game with things like promotions, events etc.
Apart from active members, choose metrics that are dependent on your specific business case.
If you have a support community, perhaps more posts is better since you provide more support (or maybe it is indicative that your documentation sucks…)
If it’s a CoP, you could look at number of (knowledge sharing) threads started etc.
What do you (and what does your boss) want out of the community?