How Many Senses Can Get Crossed?
As humans, we have five senses:
Taste, sight, sound, touch, and smell. For someone with synesthesia, there can be unlimited variants between all five senses. This allows for an entirely unique experience even among those with synesthesia; that is, there is no uniform, consistent experience. For example, someone with grapheme→color synesthesia may experience their A's as violet, but another person with the same type of synesthesia may identify their A's as chartreuse. This experience is not universal.
Theoretically, there are up to 56 different types of synesthesia that are possible. Statisically speaking, grapheme→color synesthesia is the most common form of synesthesia. This form is basically the color coding of words- in other words, texts and letter from the alphabet have a unique color associated with them.
Synesthesia Forms Chart:
(Image: http://www.daysyn.com/types-of-syn.html)
(Graph of all conceivable types of synesthesia, and their prevalence.)
(Visual Representation of what a person with grapheme→color synesthesia might experience.)
There is literally an almost unlimited amount of senses combinations that can become crossed to produce a synesthesic experience- the options are endless, and quite often, spectacular.
Another fantastic form of synesthesia is chromesthesia, the association of sounds with colors. For people who experience chromesthesia, sounds, such as a dog barking or their favorite song, can trigger vivid colors that they can 'see' spread out in front of them. Often, these colors will appear in various patterns, shapes, or styles. People with this form of synesthesia are often noted to have perfect pitch, as identifying keys or notes becomes almost simple when associated with certain colors.
(Source: Youtube)
(Video explaining chromesthesia/providing a visual representation of what someone with chromesthesia might experience.)
Other types of synesthesia include number form synesthesia, which is "...essentially a mental map that consists of numbers", reports synesthesiatest.org.
Another rarer type of synesthesia deals with taste: Lexical-Gustatory Synesthesia. This type of synesthesia occurs when certain sounds, or words, trigger a certain taste in the synethesate's mouth. For example, a coworker's name might taste vividly like grapes, while the word 'dog' could taste like strawberries. These word-to-taste experiences aren't universal however, they are as unique and varied as any other synesthesia experience.