If you want to put your money into bitcoin AND some alts, that is not enough neither, because when bitcoin goes down almost all alts go down as well, and if it bitcoin goes up almost all coins go up as well, there is rarely any difference, it is not enough at all, yet it is at least bare bottom what you can do, better than only having just bitcoin because that is riskier.
I think diversifying with alts is riskier than just holding some bitcoin. If bitcoin got bearish, say, got -10% a day, most alts would suffer more than that, and it might take a while for them to recover. Prolly longer than bitcoin to recover itself. You'd better hold bitcoin only if you want to avoid the risk and only go that way if you want to 'gamble' with some alts, hoping they could net you 2-3x in the next few months.
A better diversification would be crypto and other assets if the target is avoiding risk with moderate gains as the target.