Well, your exact question can't be answered as it stands - or rather, the answer is, you won't be able to fit that much wire on a voice coil that needs the correct impedance. The wattage question is moot because your wire can handle not only 50 Watts, but around 5000 Watts if you keep the Amperage around 9 Amps continuous.. And it can handle about 15,000 Watt spikes... But that's not really what you want to know probably.
What you need to know, is you need thinner speaker wire.
Your ~17 Gauge wire, to get it down to the correct ohmage would be about - 1000m / 13 Ohms = x/8 Ohms (if it is an 8 ohm speaker system (generally home systems)- or 4 if it is a 4 Ohm (generally, a car speaker system)).
Thus, you'd need 615m (8 ohm) or 307m (4 Ohm)..
Extremely large, It's pointless to calculate because the number of turns in the thousands and with the thickness of the wire, you won't be able to fit it in your magnet.
You can save your time winding, and money, buy using thinner copper. each halving of diameter will increase the ohmage by 4 times. So if you use 0.25mm solid wire, you'll be happy to use 16 times less length of wire. Or about 17m for 4 ohms, or 34m for 8 Ohms. And this is about what most woofer speakers use - 28 Gauge sold copper wire, about 120 feet of it, give or take.
Once you know the length of wire, and I'm just giving approximations here, you can calculate the humber of turns (and layers) required. You have to know things like, how long is the throw if your woofer (as you will need to make sure your VC length is as short or shorter than this. Also, how many turns do you need on the VC to make it fit perfectly in your magnet?
So calculating the number of turns is the least of all the problems here. Making a speaker can be fun, but if you are doing experiments - you don't need to reinvent the wheel - there are many people who have done this - and far more - and compiled these into beginner guides. Q factors, capacitance, inductance, etc. will all be necessary to understanding making a speaker and it's enclosure.