Started as a freelancer, got too many projects at the same time. Could not meet any deadline and got burned out. Everybody lose.
Build up a small dev team, got too many projects again and got a single person working on three projects at a time. Same thing: long days, too little progress, could not meet deadlines. Everybody lose.
The issue with dev work and switching between projects is that is takes a few minutes to an hour to focus and get really productive.
With experience you will find what is the equation that works for you. In my case I do not do more than one dev/code work at a given time or quality starts to degrade.
I can provide consulting, answer questions and do estimes on other projects while coding, but only one dev work at a time.
Know thyself. Find a niche and develop an appreciation for the value of your work. Only if you know what you're good at and if you have the self-esteem to say so will you be able to say no to work that doesn't suit your profile.