The number 2. It comes after one and before three.

Note: 'two', 'to', and 'too' all sound the same (/tuː/) but have different meanings. 'Two' = number 2; 'to' = preposition (I go to school); 'too' = also or very (me too, too hot).

'Two's company, three's a crowd' is an English saying meaning two people are happy together but a third person is unwelcome. 'Put two and two together' means to work out what is happening from the available evidence.