It completely depends on you because how much risk you want to take. Every person will decide according to his income so even you can start with as little as 100$ but if you are looking for long term investment better invest more into crypto currency.
It is good amount to start trading because you can get enough altcoins and you will get enough profit if you purchase to the good altcoins. Don't start with high amount because your knowledge is not enough because possible you lose some of your capital. Better to add some if your knwoeldge is enough.