Hi @Nanami Yoshizumi and welcome to the Community!
Helping customers tackle challenges like this is what I do for a living as a consultant at an Atlassian partner. Just based on your name, I suspect you're not from my neighbourhood, but I would recommend looking for a solutions partner nearby to help you with this. Services are not free, but you should be up to speed, using best practices in little time.
This advice comes from the fact that setting up a project management practice involves a combination of practices / people / tools working together nicely, which is not something we can answer in just a few lines.
Take away Jira from your question and rephrase it like this:
How can I effectively manage 300 projects simultaneously while delivering on time, making customers and our teams doing the work happy
I can confidently say - from helping many organisations with similar challenges - that the Atlassian platform has all (or definitely most of what) you need to achieve those goals, but you need to look at what your projects look like, how you define work, how you do planning, track progress, communicate with stakeholders, measure outcomes, ...
I wish I could answer that in just a few sentences, but that would not be a realistic expectation.
Still, hope this helps!
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