In general, you can run this MacBook Pro (and others) on very low wattage - because it has a battery. You do not need a 140W charger, or a 100W charger or anything like that.

However, there's a risk that you will drain the battery if your particular use of the laptop makes it consume more power than you can charge the battery with.

Depending on the particular model of the 16" MacBook Pro (2021) the maximum power consumption varies a lot. In practice, you'll see a power consumption of about ~5-15W when the laptop isn't doing much, and ~50-100W when you put it to work. If you really stress test the maxed out model while having the display set on max brightness, you will probably be able to reach a maximum power consumption of ~135W.

So in essence, for most people having a ~96W charger plugged into a Thunderbolt would be perfectly fine.

I’ve used 5 volt 2.1 amp USB-A charger with a USB-C cable for M1 Max MacBook Pro 16.

I suspect anything 5 watt plus will work.

Here is an on topic YouTube video.

To be very clear, at less than 15 to 20 watts, your battery may drain faster than it can be charged if the computer isn’t idle.

This command tells you the current power draw:

sudo powermetrics -i 1000 -s cpu_power,gpu_power | \
    egrep '^Package Power:|^GPU Power:'

You will likely lose 5 percent of the charge efficiency. In my testing while typing this, my M1 Max used about 500 mW to 961 mW. That is about 1 watt. So a 5 watt iPhone charger would maintain my current power level. If you are using significantly more power, you will want a much higher power output like the 20 watt to 96 watt chargers.

@gidds also recommended a menu bar app to determine power draw.