Can I use the power adapter of Macbook Pro 2020 to charge for my iPhone 12 Pro Max?

Apple itself has tested fast charging the iPhone 12 via MacBook adapters: https://support.apple.com/en-us/HT208137

The MacBook Pro, 13" 2000 model, is compatible with the 67W USB-C adapter, the 96W USB-C adapter or the 140W USB-C adapter.

The iPhone 12 Pro Max is compatible with the 20W USB-C Power Adapter, the MagSafe Duo Charger, the MagSafe Charger, or the MagSafe Battery Pack

Thanks caseyhb, I had no idea about this. The specs on the Apple store could do with a bit of extra information it would seem.

The 2020 MBP comes with the 61W charger which is one of those listed on the support page you linked.

Correct - the two are not compatible. If it sounds wrong it probably is - "let your own discretion be your tutor".