No, not all guitars have six strings. The guitar has evolved over the years, though the standard tuning has six strings. Now we have five, four, seven, eight, ten, twelve and even fourteen string guitars.
Electric string bass guitars mostly have four or five strings. A twelve string guitar is just like the six string guitar with 2 strings per note. The fourteen string guitar is just like the twelve string guitar with two additional sub bass strings. The eight string guitar is not as common as the ones listed above.
Therefore, not all guitars have six strings, some have less while some have more than six strings.
All guitars do not have six strings because there are variations in guitar configurations for the specific purpose of creating different sounds. Electric string bass guitars have four or five strings. There are also some twelve string guitars that are played like six strings but with two strings, per note.
A typical acoustic guitar has six strings, in order from lowest pitch to highest pitch. There are also eight-string guitars, but they are less common. Tenor guitars have four strings. Those with eight or twelve strings are generally used by fancier musicians with a lot of experience. There is also such a thing called an Archguitar which has eleven strings.