I guess you should mentioned what coin you are trying to send multiple transaction at once. Because every blockchain based token has its own tool.
Here you can check this tools. Disperse were mentioned already above. Multisender is a tool for ethereum based tokens while bulksender have it different network support such as binance, ethereum, tron, matic, AVAX and solana network.
https://disperse.app/
https://bulksender.app/
https://multisend.org/
Now if you're talking about bitcoin, then electrum is the best choice as already mentioned many times above.
Another note, this multi sender is probably uses large gas fees, because they are designed to send at least 20 transactions above or so. So expect that fees they required is high.
Disperse.app is somewhat the cheapest fees required on the list above.