The number of schemes in a portfolio depends on the requirement and investment objective of the investor.
If a particular fund can achieve a certain short term goal than one or two schemes are enough.
For long term goals, investing in different categories of schemes is recommended most of the time as in the long duration the market conditions can be different for every category.
Investing in multiple schemes of the same category does not help while over-diversifying the portfolio will restrict growth.
For me, I have chosen 5 schemes in my portfolio and they bring a great balance of risk and reward to my portfolio.