import asyncio
import time
from concurrent.futures import ThreadPoolExecutor

executor = ThreadPoolExecutor(max_workers=5)
loop = asyncio.get_event_loop()
loop.set_default_executor(executor)

def block():
    time.sleep(0.1)

async def main():
    for i in range(3):
        loop.run_in_executor(None, block)
    await asyncio.sleep(0.2)
    print("idle threads", executor._idle_semaphore._value) # 3 - correct

    for i in range(5):
        loop.run_in_executor(None, block)
    await asyncio.sleep(0.2)
    print("idle threads", executor._idle_semaphore._value) # 5 - correct

    for i in range(20):
        loop.run_in_executor(None, block)
    await asyncio.sleep(0.5)
    print("idle threads", executor._idle_semaphore._value) # 20 - wrong

    for i in range(2):
        loop.run_in_executor(None, block)
    await asyncio.sleep(0.2)
    print("idle threads", executor._idle_semaphore._value) # 20 - still wrong

loop.run_until_complete(main())