It is my first math research paper, I have no prior experience in this.
The purpose of PhD programs is to train people to do research, including the process of publishing papers. If you're an independent researcher with no prior training, you should not expect this to be easy and likely not even possible. Imagine if instead your post was "I'm an independent carpenter with no prior experience. I have some wood and I'd like to build a house; I put in some nails but no one wants to live there even though I've been handing out pictures of my wood pile to people on the city bus. Please help." I don't write this to be rude, I write this to help explain that doing and publishing research is a trained profession, and no one without training should expect they can do it without training, and that this is totally okay and does not mean you don't have potential to publish good research or talent any more than someone with interest and talent in carpentry is not expected to build houses from scratch with just that interest and talent.
I submitted my paper to multiple journals, but they found out about the multiple submissions and replied negatively.
Yes, they replied negatively because this is extremely rude behavior - reviewing a manuscript in academia takes volunteer time. You've asked a bunch of different volunteers to simultaneously spend their valuable time all on your manuscript rather than the many others they've received to review. An advisor would have helped you avoid this.
I tried to list my article on arXiv, but they require endorsement, which is hard to get.
ArXiv requires endorsement because they're trying to limit the amount of junk that gets posted on their site. An advisor could help you with both the endorsement part and with knowing whether your paper is useful or junk.
These days I have been sending about 40 emails per day to either universities professors and to authors on arXiv and ask for the endorsement.
This is also rude. Don't do this.
My article is about number theory. First year at uni math knowledge should be enough to understand.
If your article is understandable with just first year university math knowledge, it might be a good demonstration of what you understand about first year university math, but it is almost certainly not useful research-level mathematics, the kind that gets published in journals. An advisor would help you understand what sorts of papers are useful to publish and whether yours fits this category.
Can I please have an advice?
You need a research advisor/mentor. One way to obtain such a mentor is to apply for graduate programs - PhD programs and many masters programs provide mentorship in research; for PhD programs it's the main point. It may be possible to find someone to mentor you even without formally enrolling in a program, but this is also difficult - possibly more difficult than applying the normal way (why should a professor spend time mentoring someone who isn't admitted to their program, when they have other students who are admitted that also have questions?). You're more likely to convince someone to be your mentor if they appreciate your potential for research rather than this paper in particular.