Hi!
If she's happy, esp if she wakes happy and goes to sleep happy, then I wouldn't stress about it. She is getting 11.5-12 hours sleep/day and yes, whilst that is lower than average, it is definitely not that low and is/was what my LO gets/got per day.
Has she always been below average on sleep needs or is this a recent thing?
With L, who has always been LSN, we found that we could increase her total sleep/day (or keep it as high as possible) by capping her nap. That way she would nod off earlier at BT and give us a much longer night overall so her total sleep/day was longer.
At 20 months I think we capped at around 45 mins. At 24 months she was down to around 20-25 mins, and now at 28 months (if she naps) it is around 15 mins. Often she doesn't nap.
If you are worried you could try capping her nap slightly to see if that gives you a longer night. However, I know some LOs get upset if you wake them from their nap.
I also know that others have had success with routine tweaking, eg by going for a later nap, it is longer, but also doesn't seem to reduce night sleep so total sleep is increased.
Nap-capping is what worked for us so that's why I mentioned it. However, if the lower amount of sleep is a recent thing (ie she hasn't always been LSN) then perhaps a routine tweak (later nap) rather than nap capping would be better to try first.
HTH