This is a great question that I hear get asked quite often so I'm happy to offer my perspective.
There are no specific guidelines for min/max number of boards you should have. What's more important is the title/description and content of the boards.
Your boards (and the pins in them) attract your audience so only create boards related to your business or audience you want to attract. (For example if you are an interior designer, don't have public boards with recipes - keep them secret if you use them personally or have a separate personal account)
It's also a good idea to have multiple boards that a pin could go into in order to reach a broader audience. So have a few broad-topic board along with various niched-down topics. When you publish a fresh pin, pin it to the **most-relevant** niched down board followed by the other related boards (scheduled out at an interval of 7+ days or so - I use Tailwind for this).
Here's an example of what I mean:
Say you are a social media marketer, you might have boards like
Social Media Marketing
Say, you make a blog post about using Idea pins on Pinterest. Here's how I would schedule it....
Schedule it to "Pinterest Marketing" first, then 7 days later have it publish to "Social Media Marketing", and 7 days after that into "Marketing".
*By pinning to your "Pinterest Marketing" board first, you are helping give Pinterest the most context on what the pin is about which will help it get indexed in the right search results and more likely to be shown to people interested in Pinterest marketing.