target turntable shelf-how much weight

I just installed one. The WW1 I believe it is spec'd at 70 lbs capacity. You can install drywall anchors for the center holes and use studs for the outside screws. That's what I did and it is supporting my Marantz TT15 just fine. But that's only 15 lbs or so.

I'll probably upgrade the MDF board to a maple platform and use better spike hardware at some point, but it seems to work fine. Pretty easy to level it out all around and quite stable.

I played a record over the weekend and my toddler daughter was dancing around and running around to the music and it didn't skip a beat. This is installed on an interior stud wall--for best isolation it is best to mount on an exterior wall, if you can.

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")!