what is the maximum wattage that an iphone mini 13 can take through a lightening connection?

If you are asking the rating of the power source, there is no upper limit, because the phone only takes what it needs from the power source. The most it will use is 20 watts, so an Apple 20 watt “brick” will provide the shortest charge time. But you can use a 61 watt or 93 watt power source or any other USB-C/PD source also; the phone will still take only 20 watts. But only use high quality power sources, either Apple or certified as Made for iPhone from reputable vendors such as Mophie, Anker, Griffin or others in that quality class.

There won’t be much difference. The phone’s fasted fast charge is to 50% state of charge; with a 20W charger it will do that in about 30 minutes; with a 12 watt probably 40 minutes. But past 50% it slows down, dropping to 5 watts above 80%, then tapering to zero at 100%.

The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates.

If you charge overnight there is no need for a fast charge; a 5 watt charger is sufficient.