How many l's does the name have?
heard as lunullaria
spelled lunularia
Two, one in each half of Lunularia, never side by side as in lunullaria.
What the ear did
The stress falls on the syllable after the second l, which is exactly the position where English doubles a consonant. Latin diminutives do not double: lunula has a single l, and so does the genus built from it.
L-U-N-U-L-A-R-I-A
The full entry for lunularia covers the name itself.