All digital coins have a price, which is translated into terms of fiat currency (Euro, Dollar etc.). If we multiply this value by the total available amount of this currency, then we find its capitalization (market cap). Traditionally and as is easily understood, Bitcoin has the largest capitalization of all other currencies.
Flippening is a hypothetical event in which most of the capitalization is no longer owned by Bitcoin, but another currency has managed to take the largest part of the market. From time to time, several currencies have been proposed as better suggestions than Bitcoin, but none has managed to become the first choice of digital currency users. More recent "threats" are Ethereum and Bitcoin Cash, but for different reasons each: Ethereum due to smart contracts and Bitcoin Cash due to the speed and the lower cost of doing the transactions.
As much as these contingencies seem unlikely, every investor should take them seriously and along with investing in Bitcoin, invest not only in Bitcoin Cash or Ethereum but in all the serious proposals that occasionally arise, since hedging is necessary in any investment activity.
The proportion of Bitcoin Cash that an investor should have in relation to Bitcoins is mainly a matter of investor's attitude, but it would be wise for any Bitcoin that we have, to make an equal purchase in Bitcoin Cash, as this is the ratio to which each Bitcoin owner was originally credited and is a safeguard for the investor that regardless what happens, their investment will not go to zero. For those investors who in the event of flippening want to have an additional financial benefit, then the gradual expansion of their portfolio into these currencies is a necessary condition.