I used to be a strong defender of only having a very limited number of pens. My opinion was coloured by minimalism. I have come around, and I see a value in having and carrying more than one pen. It all depends on what you need.
Some people like du use different pens for different things; some use different pens for different things; while others have different fountain pens for different things. I always have three or four pens with me. I have more or less the same ink in all of them, and the nib is also more or less the same.
You might wonder why I have so many pens. I have one non-fountain pen, and three fountain pens. The reason is simple, I want to always have a pen that I enjoy to use available. I write a lot every single day, and my goal is to have enough pens with me to be able to go at least a couple of weeks without refilling.
There are many good reasons to carry multiple pens. But the most important thing is that you use them, and don't just carry a lot of pens; and don't just own a lot of pens you don't use.