Is she getting 12 hours of uninterrupted sleep at night?
Quality of night time sleep will be a big pointer to how much daytime sleep is needed. My DD would have needed significantly more daytime sleep than that at 6 months old, but she wasn't consistently sleeping though at 6 months old. Even now at nearly 14 months and 12 hours uninterrupted sleep she needs 3-3.5h daytime sleep.
At 6 months old I would have suggested three naps - 2h morning, 2h lunchtime then a 30-45 minutes powernap at teatime. Then a 7.30-8.00 bedtime.