As a secondary strain, you don't need to make a starter. I think White-Labs vials have 1/5th of the amount of Brett as they have with Sacc, and the quantity is designed for secondary pitching into 5 gallons of beer.
If you want to save some, make a small starter. Pretty much any size bottle you can fit an airlock on. I use a 1.5 liter wine bottle with one of the small bungs on it. Use some of the starter in your beer, top up the starter and store on a dark place. Provided the airlock doesn't dry out, the Brett in the starter should be good for months, and if you periodically feed it (add fresh wort to it), it can last years.