People in videos online show their character sheets for Dishonored 2, and they talk about how they have lots of bonecharms active at any given time, and that you can get more slots for bonecharms or something at black markets.

Do any of you know what they mean by this? When I look at my bonecharms in Dishonored 2, it looks like I have 5 active bone charms, but I see the game has more slots available that look like I could use them.

It says I can get more slots or something at black markets... but I was just at the black market and didn't see where you have the option to buy more bonecharm slots.

How can I equip more bonecharms?

The ability to use additional bonecharms is something you "purchase" at character advancement - early on I usually tried to have one more slot than I had useful bonecharms, in case I picked up something great. But you will always have to make trade-offs, as you can never have enough slots to use all the charms you acquire (if you search for them, anyway).

Note that you can swap a bonecharm in just before needing it, to help with particular tasks.