The number '2' refers to the number of tones used to encode the signal. For example, 2-FSK is essentially sending binary data using two frequencies. One symbol (time slice) has the potential for two values only. 2-FSK is usually what is meant by an unqualified mention of FSK.
It's possible to use more than two tones. These modulation schemes usually are called MFSK for Multiple Frequency Shift Keying. A common MFSK scheme in amateur radio is MFSK16, which uses 16 different frequencies to represent 4 binary bits in each symbol. By your notation, this would be 16-FSK.
The more frequencies that you use in a symbol, the more complicated your modulation/demodulation circuit. Typical MFSK applications in amateur radio require a computer sound card and CPU for signal processing. Analog circuitry can be developed somewhat easily for FSK using two band-pass filters.