This is the most common question i get from people when they want to start investing. The real answer is there is NO fixed amount. You should only invest the money you are willing to risk and that entirely depends on your risk appetite.
You can separate your investment into a lump sum amount and a fixed amount every month. Make a portfolio of 4-9 tokens depending on your capital and DCA (Dollar Cost Average) into the tokens. This means you purchase a fixed amount every month which will allow you to lower your average purchase price.