socket.io-redux | Redux middleware to emit action via socket.io | Socket library

I have done the exact same thing in a previous project that I worked, saved in an array the mongo and socket id-s of each user.

Think of it this way, how much memory will one of these objects take, about 50 bytes maybe? 60? So technically you can have ~10 million of these in memory, assuming with a maximum usage of 500 MB of memory. The Node maximum memory limit goes from 700MB to 1.4GB between 32 and 64 bit systems, so with 10 million socket instances (which are not the same as 10 million simultaneous users since a user can have more than one tab open, and one socket is created for each tab/instance) you should be safe, since Node itself is assumed to not be used for intensive operations.

Now these are the limits, and in the end, you still can stretch beyond this limit by using some Redis or Memcache to be sort of in between memory and storage speed, and since I don't know your scenario I assume that you are pretty safe in terms of leaks if you correctly cleanup the logged out users.