How do you select promising coins?
I was selecting the promising coin based on on the fundamental analysis, it should be from the strategic partnership, future roadmap, the result of product, hype from the community and many more.
How many coins should be in an investment portfolio?
This can't be determined coz that depends on how much coins that makes me feel interested.
How do I buy, all at once or in part over the course of a month?
If you are wanna decreasing the risk for your investment and you should buy that on the various rate.
How to deversify risks and rebalance ?
Never put your all of your money in the same basket. That will be putting yourself into the big risk. Try to divide it into the various kind of assets.