Is it recommended to have multiple SQL servers in a farm to host content databases or usually just one? sorry if this is a basic question but i am new to the SP architecture
Yes, You can have more than one SQL server in your farm for content as well Services DB. But it is depend upon your requirement i.e amount of data you have, number of user and daily traffic. Usually customer separate the config db from Content DBs, spread Content on multiple instances and separate the Service DBs as well. But again, depend upon your farm's requirement how big it is.
But one thing, MSFT always recommend that put one SQL instance on one server. We had two instance on one server which give us performance issue. Some resource extensive services like search will cause trouble if you have more than one sql Instance on a server.
You can add more database servers at any time to respond to business or operations requirements. Because a database server contains the farm content, which can consist of diverse types of data and can have a fast growing document collection, the size of the farm databases can grow quickly. Storage capacity is often the key reason to add more database servers. Other reasons can include adding new features, improving performance and high availability.