How many power outlets should a kitchen have at minimum?

How many sockets?
Think about how many devices you need simultaneously during an extensive cooking and baking event in your kitchen and where they might be located. Then add one extra socket as a buffer at each of these locations. Next, check if there is any work area without a socket in the plan and simply add one there as well.
We only have 6 sockets in three places in the kitchen and manage perfectly fine with them, but we don't cook very "appliance-oriented." The only interchangeable devices are a food processor, a blender, an immersion blender, and an egg cooker (which annoys me). The coffee machine and coffee grinder are permanently plugged into one socket. For charging small devices ([Grillfleischthermometer], flashlight, smartphone, tablet, etc.) there are 4 USB ports hidden in 2 of the sockets inside a shelf compartment, since it is practical not to have those things on the countertop. Sometimes something gets unplugged and plugged back in.

Hi. I am also currently dealing with this topic. Since we are redoing the tile backsplash, we are still flexible. We would expand the existing single or double sockets in the corner (U shape) by 1 or 2 sockets. However, I wonder if multiple sockets for the corner might not be more practical in the end. That way, you wouldn’t have to install extra sockets (at least not in the corner). I’m still a bit undecided about this... for the corner solution, you would probably have to drill through the countertop from below. At least, that’s how I imagine it.

Thebo flush-mounted sockets are recommended, also available with integrated light switch.

I mean the free sockets that are above the countertop for the movable devices (coffee machine, kettle, blender, etc.

No sockets above the countertop. Put them all into the countertop. That way you won't spoil the wall. How many? Depends on the number of devices that are permanently installed and the size of the kitchen. On/at the countertop, I would plan at least 6 pieces. For example - there are sockets with USB and/or LAN

Let them all go into the countertop. That way you don't ruin the wall.

No, but the countertop instead. Takes up unnecessary space on the countertop and is also a dirt trap. Not to mention the rather expensive solutions.

Apart from the fact that many devices are permanently plugged in, such as coffee machines, kettles, etc., making them retractable makes no sense.