from multiprocessing import Process, Pool
import time


all_task = []
thread_pool_count = 0

def abc1(i, j):
    time.sleep(20)
def abc2(i, j):
    time.sleep(10)
def abc3(i, j):
    time.sleep(5)

#thread_pool = ThreadPoolExecutor(10)
thread_pool = Pool(processes = 10)

def print_error(value):
    print("Process pool report error: {}".format(value))

i = 0
j = 0
thread_pool.apply_async(
    func = abc1,
    args = (
        i,
        j
    ),
    error_callback = print_error
)
thread_pool.apply_async(
    func = abc2,
    args = (
        i,
        j
    ),
    error_callback = print_error
)
thread_pool.apply_async(
    func = abc3,
    args = (
        i,
        j
    ),
    error_callback = print_error
)

while True:
    print(len(thread_pool._cache))
    time.sleep(1)

结论：

print(len(thread_pool._cache))

打印进程池内正在运行的进程个数