From my experience it all depends on you and if you are able to keep up having a personal Instagram and promote on a separate one.
The problem with just having a personal Instagram is that you might feel conflicted about posting a particular photo because you feel like it might ruin your feed, especially as a starting youtuber people might care more about your content more than your personal life. I tried to have a separate account for my old channel, I thought it was a good idea because people cared about my content, I was gaining followers but it was too difficult for me to post everyday on my personal and separate channel. I now have one account and I do different videos on a different channel but knowing my subscribers are following my personal account I don't feel as free to post things as when I had a separate account (I also feel embarrassed that my high school friends from years ago see me trying to promote my youtube).
Hope this helps you decision!