If the Piano is in C, then you would be in A.... (three sharps)
Here is the breakdown of transpositions from piano to sax :MAJOR KEYS
Piano Key ALTO SAX TENOR/SOPRANO SAX
C Major A Major (3 sharps - F#,C#,G#) D Major (2 sharps - F#,C#)
G Major E Major (4 sharps - F#,C#,G#,D#) A Major (3 sharps)
D Major B Major (5 sharps - F#,C#,G#,D#,A#) E Major (4 sharps)
A Major F# Major (6 sharps - F#,C#,G#,D#,A#,E#) B Major (5 sharps)
Gb Major (6 flats - Bb,Eb,Ab,Db,Gb,Cb)
E Major C# Major (7 sharps - F#,C#,G#,D#,A#,E#,B#) F# Major (6 sharps)
Db Major (5 flats - Bb,Eb,Ab,Db,Gb) Gb Major (6 flats)
B Major Ab Major (4 flats - Bb,Eb,Ab,Db) C# Major (7 sharps)
Db Major (5 flats)
F#/Gb Major Eb Major (3 flats - Bb,Eb,Ab) Ab Major (4 flats)
C#/Db Major Bb Major (2 flats - Bb,Eb) Eb Major (3 flats)
Ab Major F Major (1 flat - Bb) Bb Major (2 flats)
Eb Major C Major - NO FLATS or SHARPS F Major (1 flat)
Bb Major G Major (1 Sharp - F#) C Major
F Major D Major (2 sharps - F#,C#) G Major (1 sharp)