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.