It depends what you want to achieve. When would you like to access the capital?
There were various types of diversification:
Asset classes - shares (equities), corporate bonds, previous metals (such as gold), government debt, etc.
Sectors - defence, tobacco, oil, tech, pharmaceuticals, etc
Regions - Global, North America, Europe, UK, Asia, China, Japan.
Themes - infrastructure, automation, aging population, etc.
If you're investing longer term (10+ years) then equities have historically provided the best returns, but can also go through periods where your portfolio is underwater.
If you're happy to accept the risks associated with equities, then it is very simple to get diversity by purchasing a single share in a global tracker fund.
This vanguard fund has an expense ratio of 0.06%
https://www.tipranks.com/etf/gb:0lmn
"Vanguard Total World Stock ETFThe Vanguard Total World Stock ETF (Ticker: VT) is a comprehensive investment vehicle that offers exposure to the entire global stock market in one convenient package. Designed for investors seeking a diversified, one-stop solution, VT captures the essence of the total market approach by including a broad array of equities from both developed and emerging markets across the globe. This ETF falls under the 'Size and Style' category, focusing on the Total Market, making it an excellent choice for those aiming to achieve a balanced portfolio without the need to manage multiple individual investments. VT stands out in its broad-based niche, providing unparalleled access to over 9,000 stocks, thereby encompassing large-, mid-, and small-cap companies. Investors benefit from the fund's ability to mitigate risk through diversification while still participating in global growth opportunities. With a low expense ratio, VT is not only cost-effective but also strategically structured to track the FTSE Global All Cap Index, ensuring comprehensive representation of the worldwide equity market. Ideal for long-term investors, the Vanguard Total World Stock ETF is a gateway to global diversification, offering a straightforward means to invest in the world's economic engine, from the United States to the farthest corners of the international market. Whether you're an individual investor looking to expand your portfolio or a seasoned financier seeking a global equity strategy, VT provides a well-rounded, efficient, and engaging investment opportunity."
When you buy one share in that fund, you're actually buying a stake in 4,889 companies.
www.tipranks.com/etf/gb:0lmn/holdings