target turntable shelf-how much weight

I use one with a Garrard 401 in a heavy plinth, and I've switched out the MDF shelf for a heavy maple platform as well. The way I've ensured stability is to attach the Target steel shelf to a hardwood board (secured in all six of the connections), and then I attached the hardwood board across three studs.

I used a red oak solid plank 23" by 12" by 1/2" and was able to span three studs by starting alongside a window molding. It looks great since I stained the hardwood backing a similar color to the maple shelf and the plinth. I may not have done it if I had to make it 34" wide (i.e. the span of 16" studs x 3), but I was lucky to have the window in the right place. A 34" wide board would not have looked nearly as good.

You should be fine as long as you use a good stud finder and hit the center of the studs, and also use wall anchors for the center connections.

There is one way to utilize the other mounting holes on your shelf if you are worried about the load. You could screw together a wood frame, mount it to the wall studs with heavy screws, and attach the shelf to it.
I have the older Target shelf, it has different frame with more support and six mounting points on the outer sides. In my old house I mounted it near a room corner where the studs were closer than 16" and used this method.

That's a very good idea about using the oak board as an attachment device. I may go that route some day, when I really feel like tweaking (and if I can get the screws back out of the wall--I used long stainless deck screws to make sure I hit the "meat")!