Multithreading in python: how to hold the code inside the for loop and limit 20 threads in one go

Question

I am working on a project for Algo trading using zerodha broker's API. I am trying to do multithreading to save the costly operation of calling the API function for getting historical data for 50 stocks at a time and then apply my strategy on it for buy/sell.

Here is my code:

1. historical data function:

def historicData(token, start_dt, end_dt):
   data = pd.DataFrame( kite.historical_data( token,
                                              start_dt,
                                              end_dt,
                                              interval   = 'minute',
                                              continuous = False,
                                              oi         = False
                                              )
                        )
   # kite.historical_data() is the API call with limitation of 20 requests/sec
   return data.tail(5)

1. Strategy function:

def Strategy(token, script_name):
    start_dt = (datetime.now() - timedelta(3)).strftime("%Y-%m-%d")
    end_dt = datetime.now().strftime("%Y-%m-%d")

    ScriptData = historicData(token, start_dt, end_dt)
    # perform operations on ScriptData
    print(token,script_name)

3. concurrent calling of the above function:

# concurrent code
from threading import Thread, Lock

start_dt = (datetime.now() - timedelta(3)).strftime("%Y-%m-%d")
end_dt = datetime.now().strftime("%Y-%m-%d")

th_list = [None]*10

start = sleep.time()
for i in range(0,50,20):             # trying to send 20 request in a form of 20 threads in one go
    token_batch = tokens[i:i+20]     # data inside is ['123414','124124',...] total 50
    script_batch = scripts[i:i+20]   # data inside is ['RELIANCE','INFY',...] total 50
    j=0

    for stock_script in zip(token_batch,script_batch):
        th_list[j] = Thread( target = Strategy,
                             args   = ( stock_script[0],
                                        stock_script[1]
                                        )
                             )
        th_list[j].start()
        j+=1


end = sleep.time()
print('time is : ', end-start)

Now there are 2 issues I am unable to resolve after 2 days of trying many solutions online.

1. There's a bottleneck from the API server that it accepts 20 API calls per second and rejects if more are called. Total stocks in the list are 50 and I am trying to do is get data of 20 stocks at a time then get another 20 in the next and then remaining 10 in the third go. Total stocks list is gonna get big with 200 stocks soon that's why a serial execution is too costly for my strategy to work.
2. When running this function concurrently, there are too many threads created at once and API request exceeds... and print('time is : ', end-start) runs as soon as I run the 3rd cell.

So how do I block the code from leaving the inner for loop before all threads finish their execution.

and

Is my way correct to get 20 threads at the most per second? Should I place a sleep(1) somewhere?