It all depends on yourself as a writer. Personally I’m working on two regularly updated stories at the same time. [Plus two other less regularly updated, so that makes it 4.]
Plus six other unpublished stories. [So ten in total] And although it’s very hard work… I manage. Even whilst giving regular updates for certain published stories.
I think it’s because when my motivation for one story lacks… I then have 9 other stories to choose from. Which means I’m not giving myself long breaks from writing, but instead pouring my energy [and any free time I get] into my other works in progress… and this tends to prevent writers block.
So if you’re managing okay, then I say do it however you like. But if you find yourself struggling to keep up… then maybe you’d be better off finishing the first one before starting the second. But only you know yourself well enough to understand what you need. 
And don’t worry, you’ll find your flow… different things work for different people.