Number of dumbbells I am looking at a dumbbell that can be changed in weight as a gift. However it is sold as a single dumbbell with adjustable weights. Does it make a difference whether you have one or do I need to buy two of these dumbbells.
The technical answer to your question is "no", you do not need to buy two of these dumbbells. However, I highly recommend that you do buy a pair. Although there are a number of dumbbell exercises that can be done unilateral (one side/arm at a time), there are many more that are bilateral (both sides/arms at a time). I believe having only one dumbbell will restrict the range of exercises your gift recipient can perform, thus restricting the potential benefit that comes from weight training.
Depending on how strong your gift recipient is, the SelectTech 552, which is sold as a pair, may be sufficient maximum weight (52.5 lbs).