There is no simple answer or %, it depends entirely on your own personal situation and risk tolerance. For example, many people choose to have 100% of their portfolio in BTC, or someone might have a very high-risk tolerance and choose to allocate a greater % to riskier investments. To minimise risk, many investors choose to only allocate a small % (1%) on ‘altcoins’ (i.e. coins that aren't BTC).
It may also be useful to think about what you would do if the allocation in a specific altcoin went to 0. Given the volatility of cryptocurrencies, this is a possible outcome. Ask yourself, how would you feel? A good rule of thumb is to only invest capital that you're prepared to lose.