The best cryptocurrency for your portfolio will depend on your unique situation. Before you dive into cryptocurrencies, take some time to understand the risks that come with this inherently volatile digital asset. 
The short answer is that there are hundreds of cryptocurrency exchanges. But with more created on a regular basis, it is difficult to confirm a hard number. 
As of early 2021, there are more than 4,000 cryptocurrencies. However, it is expected that the number will rise over time. 
A decentralized cryptocurrency exchange will connect buyers and sellers in a peer-to-peer format. The goal is to operate without a central authority present.  In these platforms, the user will maintain control of their private keys to access cryptocurrencies. 
Although cryptocurrency is a volatile investment with inherent risks, you can make the process itself more secure by choosing a reputable exchange. You should feel secure working with any of the companies on this list. 
You don’t need thousands of dollars to get started in cryptocurrency. However, I would recommend creating a solid emergency fund before investing too much into this volatile asset. 
Yes. It is absolutely legal to buy cryptocurrency.