How hard is it to become proficient in weapons based martial arts compared to physical fighting ones?
And would you recommend training in one weapon at a time or in more and how many?
I think it's really going to come down to how you define proficiency. And I know that I'm not the first to say that in this thread. Likely, I won't be the last. But I'm not trying to be pithy. You have to establish for yourself a metric by which you'll judge proficiency. Or find one that's been established that you can accept.
At the end of the day, that's all we have. Whether it's a belt system, a competition format, etc., all we can really do is identify a metric by which to judge our work.
I went from taekwondo to Filipino martial arts personally. I went into a system that featured a belt system and worked my way through it. I competed in the WEKAF format that took a bit of a beating up thread. I earned a rank with a colour and a name (Lakan Guro). I studied other styles of FMA and incorporated them into my practice. I studied other related arts (e.g., Western fencing, JKD, etc.). But I have never, for all that, landed on a satisfactory definition of proficiency. Just a shifting and evolving series of metrics with which I'm comfortable, and the priorities that guide them.
What do I mean by the priorities? Well, when I was younger, I probably felt more confident of my chances in a knife fight. I was almost certainly wrong, but I felt more confident. Now, I view that weapons training as a personal challenge rather than a realistic preparation. And, ironically, in doing so, I might be more likely never to get stabbed with a knife.
How many weapons you train with at once depends as well. In FMA, there are certain baseline mechanics that enjoy commonality regardless of the weapon. (Yeah, edged and blunt weapons are fundamentally different and that's valid.) So in FMA, it's not unusual to learn stick along side knife, for instance. It's not uncommon to apply the same principles to the stick and to the machete, even though there are other principles that won't carry from edged to blunt and vice versa. Regardless of all that, the number of weapons you can effectively train is going to be dependent on the level of crossover. To what extent are you training staff concepts even as you're actually training stick techniques?
Some styles pointedly have very different approaches from one weapon to another. The jian in Chinese martial arts is very different from the dao (broad sword). Intentionally so. So the crossover might be less than in FMA, where commonality in principles is a stated priority.
That same logic holds when you think about going from empty hand to weapons. A kerambit has specialized applications, but at the same time, if you were a strong boxer, you'd be really dangerous with a kerambit in each hand. But put a polearm in your hands, and you might be as big a danger to yourself as anyone else. It just depends.
You set metrics for yourself. But your level of training and experience will inform your metrics. As will your priorities. For me, learning a form (kata, sayaw, whatever) isn't going to make me feel proficient. I'm going to want to know about application. Then you need a sparring format of some sort. And if you master that, there's always the argument about rules not reflecting reality. So how far are you prepared to go in determining proficiency? Hopefully not to the logical extreme.
So explore your priorities, learn your lessons, establish your metrics, and test yourself. That's all there is, really.