A bass guitar typically has four strings. However, there are also bass guitars with five, six, or even more strings, which are often used for extended range or specific musical purposes. The four-string bass guitar is the most common configuration and is tuned to the same pitches as the four lowest strings of a regular guitar, but one octave lower: E, A, D, and G. 

Why Are There Different Numbers Of Strings?  

Some basses have more strings than others! The original 4-string 'bass guitar' popularized by Leo Fender in 1954 added frets to the upright bass and made it something you could stand and play like a guitar. He wasn't the first, but he was the first to have a hit on his hands with it. In time, people wanted to add more range than that of an upright or double bass and started adding lower strings (much like the guitar with its 7, 8, 9, string cousins).  

That original bass was tuned like the lower 4 strings on a guitar but an octave lower. In time, the 5 string added either a high C or a Low B. The modern 5-string tuning is almost always the BEADG variety. Eventually, the 6 string popularized by Carl Thompson added both of those options for a BEADGC tuning at once using D'Addario strings. 

Modern metal and other genres have allowed a lot of experimentation to bring the range of these instruments lower and lower with may have more strings than 6 and notes way lower than. But you'll likely never see this on the store shelf so when we talk about standard tuning, these are the tunings people think of.