When building a portfolio of ASX investments, investors can go down a few different routes. There's always the traditional path of building a portfolio consisting solely of individual ASX shares. But these days, many investors like to opt for other investment vehicles as well such as managed funds or ASX exchange-traded funds (ETFs). 

One can always use a combination of these vehicles to build a portfolio. However, it is increasingly common for investors to build a portfolio using only ASX ETFs.

There is nothing inherently wrong with this strategy, of course. After all, most ASX ETFs represent investments in an underlying portfolio of shares.

But exactly how many ASX ETFs should an investor aim to have in their portfolio? That's what we'll be digging into today.

How many ASX ETFs should one have in a portfolio?

Here at the Motley Fool, we normally advocate that the typical ASX investor should have between 15 and 25 individual stocks in a share portfolio if that's the path they wish to take. This ensures adequate diversification but not so much that it waters down the potential outperformance of your best shares.

When it comes to ASX ETFs, though, it's a different kettle of fish.

You could theoretically have a properly diversified portfolio using just one ASX ETF, as long as it is a broad-market index fund like the Vanguard Australian Shares Index ETF (ASX: VAS). 

An index fund like VAS delivers exposure to hundreds of underlying companies (300 in VAS' case), spanning all corners of the economy and market sectors.

However, even if we ignore how an ASX index fund will still be heavily tilted towards banks and miners, I still think it is better for most investors to branch out from one index fund. The Australian markets are of high quality. But they still represent exposure to one, relatively small, country in the global economy.  

As such, I think adding at least one more index ETF to a portfolio is a good idea for most investors. That's enough to give an investor an additional diversification boost, adding exposure to companies outside Australian jurisdiction. Not to mention outside the Australian dollar.  

You could always go with the Warren Buffett-endorsed S&P 500 Index (SP: .INX). The S&P 500 is an index that tracks the largest 500 stocks listed on the US markets. These include the likes of world-dominating stocks such as Apple, Amazon, Microsoft, Google-owner Alphabet, and even Buffett's own Berkshire Hathaway. Adding the ASX's iShares S&P 500 ETF (ASX: IVV) to an ASX index fund like VAS would produce a diversified and high-quality portfolio that, in my view, would suit most investors.

Alternatively, investors could use an even broader index fund like the Vanguard MSCI Index International Shares ETF (ASX: VGS). This fund still offers access to US stocks but also offers exposure to companies from other advanced economies like Japan, the United Kingdom, Canada, and France.

Foolish Takeaway

Not all ASX ETFs are equal. However, I think most investors would need only combine one broad-based ASX index fund with an index fund that covers international shares to pursue a passive, ETF-only portfolio.

You can always add more ETFs, though. For example, you can access emerging markets via a fund like the Vanguard FTSE Emerging Markets Shares ETF (ASX: VGE).

Alternatively, you could add a fund like the BetaShares Global Cybersecurity ETF (ASX: HACK) if you want to combine index investing with strategic exposure to one sector. 

There are no absolute answers here. But as long as you've got some level of diversification, you're on the right track.