Hours of sleep (baby not mine!)

My 5.5 week old who is such a happy, easy going baby only sleeps in total 12-13 hours per day. I'm told that isn't enough which probably makes sense given I notice she does get fussy when it's been too long since a nap. How many hours do your babies sleep and any advice on how to get her to sleep more? I put her down for a nap (try to) every hour or so. She just fights the nap sometimes!