This is the way I understand it... I'm using Vanguard as the example, although there are equivalent index funds available from other investment houses.
There are total stock market indexes like VTSAX, which is 100% stocks, and covers every company currently traded in the stock market. This is the one that is mostly recommended, since it has some of every single company on there so it about as diversified as a single fund can be.
There are total bond index funds like VBTLX, which hold the bond market, or REIT index fund (VGSLX). There is also a total international index fund: VTIAX. There are many other index funds that have a higher concentration/weight of specific funds like the Vanguard 500 (VFIAX) fund - containing the 500 largest U.S. companies.
If you're looking to have a higher concentration of certain funds, it might make sense to add in other funds that have higher concentrations in the companies you're wanting, but me personally, I think holding several different funds in addition to a total index fund in the same category (like stocks) isn't necessary, since you're basically already holding everything by being in the total stock market (VTSAX).