First of all I advise against writing in
all bold text because it hurts the eyes especially at night.
Now as for your question, I am guessing that you are looking for a library, and not some web service, for creating millions of private keys.
If you are OK with using Python, you can install the
bitcoinaddress module and simply generate an address like this:
from bitcoinaddress import Wallet
wallet = Wallet()
print(wallet)
Example output addresses: DO NOT USE
Private Key HEX: 03902e4f09664bc177fe4e090dcd9906b432b50f15fb6151984475c1c75c35b6
Private Key WIF: 5HqrbgkWPqBy6dvCE7FoUiMuiCfFPRdtRsyi6NuCM2np8qBZxq5
Private Key WIF compressed: KwLdv6T2jmhQbswnYrcL9KZHerTpVyjozp1JNjfP5QuD3GchCwCc
Public Key: 04c5389a31ce6149c28ba20d14db8540b2319e5a65000a2919fbf7a6296e7840b53f883a9483fb7f2b43f3eacd857c904d1b70ecc168571b64d8f1ab82b57eea88
Public Key compressed: 02c5389a31ce6149c28ba20d14db8540b2319e5a65000a2919fbf7a6296e7840b5
Public Address 1: 1Bu6YxH64nfvhdDsYNEP8PftoBMqgusdPS
Public Address 1 compressed: 18i5PtPisxbGiGGEviW7HPcnfNPmcsscwH
Public Address 3: 38dRrGx5YbrnRWuWcJv5i2XHjYUnHE2wvv
Public Address bc1 P2WPKH: bc1q2jxe5azr6zmhk3258av7ul6cqtu4eu4mps8f4p
Public Address bc1 P2WSH: bc1qdveuf0egtfdnd2fnsp0lzfukn2e58czf8323ky6xt8ydew4ecfcqv3ettx
You can run that in a loop and it's never going to rate-limit or block you, because it runs on your local machine
If you are looking to use some other programming language, please tell us which one it is so we can help you specifically about it.