+1 I think that many of the biggie's target date funds can be used as a starting point.
Vanguard's target date funds include domestic equities, international equities, domestic bonds and international bonds.. so a minimum of 4 is a decent place to start (though I know some people would question the need for international bonds and would go with 3).
We have 9. Our core 7 cover domestic equities, international developed equities, emerging market equities, domestic investment-grade bonds, domestic high-yield bonds, international developed bonds and emerging market bonds.
The 8th is a pesky position in DW's taxable portfolio in a S&P 500 index fund that has appreciated so much that I don't sell it for tax reasons and the 9th is a Blackrock target date investment-grade bond fund when all our others are Bulletshares.
For purposes of the above I have considered any maturity date bond fund investments as 1.... IOW, if I hold multiple years of investment-grade Bulletshares I view them as 1 rather than 3 or 5 or however many there are.