When the price is high it's hard to multiply it, let's say Bitcoin price is $30,000 the double is $60,000 it's has large difference of the price $60,000-$30,000 = $30,000.
While altcoins price is $0,2 the 20x is $4 only, the difference is only $4-$0,2 = $3,8, it's why altcoins price is easy to multiply rather than valuable asset (e.g. Bitcoin) but the risk is very high rather than Bitcoin.
For the advice you can read above user explain especially post from @NeuroticFish