Most CustomFit patterns, by default, use a yarnover buttonhole. This buttonhole works for smaller button sizes and is the simplest to work. Simply yarnover, then knit two together to form your buttonhole.
You can absolutely change the buttonholes to suit your preference, though! Many knitters like the one-row buttonhole. Some prefer to work their buttonholes over two rows. Both are great options.
Keep in mind that knit fabric tends to stretch over time, making buttonholes grow larger. Err on the smaller side to ensure that your cardigan will stay buttoned!