We bring about 20 with us. I would recommend to anyone that hasn’t purchased them yet to avoid the ones that only hold 5 pounds. I think our second set is rated for 10 pounds. Even then, we double up for heavier items.
We haven’t traveled with kids in years but still find them useful. Umbrellas, hats, medallions, binoculars all hang on hooks until we need them. Out of the way but in sight so we don’t forget them.
If you watch a video on how ships are built, each cabin is a metal module that is slid into place. I haven’t seen an outside wall that isn’t steel, and therefor holds a magnet. The only wall I’m not sure of is the one separating the closet from the “bedroom”. We just use the wall opposite the bed.