Answer: As many as you can run well.
If you can only run one social media account well, only run that one. Once you can run one well, add another. Go deep before you go wide. Become a practitioner, then scale.
For Efficacy Clothing Co. alone, I personally run the following:
And I run them well.
Now, digital/social marketing is my profession, so it comes fairly easy for me (because I live and breathe it). But keep in mind, Efficacy is my side hustle. I have a full-time career; I have a wife and three young kids; I'm a fitness instructor; I'm a soccer and baseball coach; I'm a teaching pastor; I have friends; I have hobbies; I have a life; I also sleep sometimes.
So "not enough time" usually doesn't cut it with me. Reaching customers with the message of your goods and/or services via social media is a must in the modern marketing landscape, but not at the expense of quality.
Do as many as you can do well.
That's the only answer.