What are the Jmeter threads and how many threads it can handle?

Jmeter Threads: Jmeter threads are used to execute the test plan entirely and it is independent of other threads. They indicate a single user, interacting with the application by sending them requests. When multiple users send multiple requests then that is called the Thread group.

Apache Jmeter has the capability to handle multiple threads in the same box and it can generate up to 200 threads per Jmeter instance. If in case a user needs more than that then, in that case, I would suggest you use multiple Jmeter instances. For your information, a modern machine can easily generate 500-1000 threads.