Is having 15-20 mutual funds in a portfolio a mistake?

Yes — classic diworsification. You end up owning 200+ stocks with massive overlap and no real diversification benefit. Pick 5-7 quality funds maximum: 1-2 large-cap, 1-2 mid or small-cap, 1 flexi-cap, 1 debt, optionally 1 international. Simplicity beats complexity every time.