Here's a link to a chart showing the "Standard" rim widths you could use with most any brand of racekart tires:
http://www.russellkarting.com/range.htm
The 'Range" column on that chart lists the Optional (narrower & wider width) rims that would also fit and work with each size of tire. Sometimes kart racers choose to use narrower or wider rims to alter the sidewall-flex characteristics and/or to slightly alter the circumference of the installed tire. But I'd suggest you just use the "standard" width column to select your rim widths.
You'll find that Racekart rims are also sold with various different 'offsets". You typically just want to select "center-offset" (or as close to that as available), unless you want to alter (narrow or widen) your karts overall track-width for clearance issues, or to alter the handling characteristics of your kart.