Plural nominative is «лимо́ны».
With numbers, we use the following forms:
- Nominative singular (лимо́н) is used with numbers ending in 1 (but not in 11).
- Genitive singular (лимо́на) is used with numbers endind in 2, 3, 4 (but not 12, 13, 14).
- Genitive plural (лимо́нов) is used with all othe other numbers, i.e. with numbers ending in 0, 5, 6, 7, 8, 9, 11, 12, 13, 14.
There is no real answer to ‘Why?’. We just speak this way. Long ago, Slavic languages used to have dual number. It still exists in Slovenian, but Russian lost it and replaced with forms that sounded similar to it (genitive singular). Later, the number became re-interpreted as a form for 'small numbers' (2, 3, 4), not just for 'two'.