how many Watts to charge a MBP 16inch?

It will charge your Mac, but more slowly and your Mac will sometimes use battery power even while connected to the monitor.

Apple does not recommend using a lower wattage charger than what comes with your Mac.

You can connect your Apple charger and the display, the Mac will use the Apple charger for power.

Over time, probably slightly. IMO, it's fine for occasional use (e.g. I went into the office a few weeks ago and used a 61 W charger for my 15" MBP which takes an 87 W charger. But I would not do it as a routine use.

I don't know what the minimum is but these two things may be of interest if you aren't already aware:

The current 16" MacBook Pro comes with a 96W power adapter. See the "In the box" section of MacBook Pro 16-inch - Technical Specifications - Apple

The 61W adapter sold by Apple is not listed as compatible. See the Compatibility section of: https://www.apple.com/shop/product/MRW22LL/A/61w-usb-c-power-adapter