Is there any formula to get number of windings for a 50W voice coil of a speaker?
Calculate the length of the wire required first by its resistance. If 1000 m per  13 ohm, that simply means 500 m per 6.5 ohm or 76.92 m per ohm. Now if you are wiring a coil for 4 ohms, you need 307 m wire on the coil. Now calculate the length of one turn of wire on the coil. If one turn is 2 inches, for example, then you need 2000 turns for 4 ohms impedance.

The coil is designed on ohms not watts. Watts indicate the capacity or load a coil can handle, which depends on the gauge of wire. And before wiring, you can check gauge and watt chart for adequate watt calculation.