Concurrency is the number of requests you can send through your proxies simultaneously. By default, your account comes with 500 concurrency (threads). However, if you need more, add the High Concurrency additional feature to your plan and increase the limit to 3,000+.
The number of concurrency for High Concurrency plans scales with the number of proxies available in your account. So, when you enable high concurrency, you'll receive the following concurrency based on the number of proxies:
100 proxies - 3,000 Concurrency
1,000 Proxies - 10,000 Concurrency
10,000 Proxies - 30,000 Concurrency
60,000 Proxies - 60,000 Concurrency
It's important to note that all concurrency limits apply to your account. That said, it is technically impossible to identify the exact number of concurrency on your account. Still, as explained here, your concurrency limits will automatically increase with the number of proxies you purchase.