Sure you can make your own bike, as long as you have all the right tools, equipment, and pieces to construct your bike.
2
Yes, it is reasonable to own bike insurance if you use your bike a lot in a well populated place, or if your bike is expensive. You can never be too careful.
one
Normally, two. There are usually only two seats on a tandem bike.
1,000,000 not good
It depends how you are going to use the bike. If you are determined to use the bike for many years to keep yourself in shape, it would be a good investment to buy your own. If you just want to lost the baby-weight and stop using the exercise bike, it's better to join a gym.
4 people
too many
2 if iits is an normal one
Preferably only one at a time.
14