Question: Does my laptop battery die faster if I have a lot of devices plugged into the USB ports?
Answer: Regular USB flash drives will have little impact while plugged in and not in use. Constant copying across will have some impact, but nothing major. Even a USB mouse will not degrade performance noticeably.
However, when you start relying on your laptop battery to power any peripheral that has a motor, while itself not plugged into power, expect it to drain fast.
One of my colleagues uses a USB-powered mini air-conditioner, and has learned never to use it during loadshedding or when the laptop is not plugged in.
When asked a similar question on the Microsoft Community forum, a community member said that using an external keyboard and mouse does not differ from the amount of battery used when you use the inbuilt keyboard. He also said it was not necessary to disconnect the external devices to save the battery.
While I agree with his advice that it is more effective to put the computer to sleep when not in use, to reduce power consumption, I would recommend unplugging larger peripherals when they are not in use. Especially during loadshedding, every minute of additional use of a laptop is valuable.