Not at all! In fact, just one demat account is sufficient to hold all assets that have signed up for demat and have been allocated a unique ISIN number. A single demat account can hold multiple asset classes. Your demat account not only holds your shares but also other asset classes like bonds, ETFs, RBI gold bonds, mutual funds etc. These investments are all identified through unique ISIN numbers and it is possible to get a comprehensive and central view of all your holdings. You do not have to worry about multiple folders and multiple lockers to maintain all your investments.
Everything is right there at a single place.
Now, it is not only ETFs, bonds and equities that you can hold but also open ended mutual funds. You just need to give a request for the dematerialization of your mutual fund units and they will be converted into demat credits with the unique ISIN mapped to the units. It actually makes your net worth measurement much easier.