Not only am I naming a soon-to-be daughter, but also the character in my latest novel attempt! She’s tall, has a younger brother called [name]Isaac[/name] Epvard, and needs a name. [name]Octavia[/name] is possible, I love [name]Tavi[/name], but she isn’t an eighth child. Something long and regal with loads of nickname options would be ideal.
[name]Phillipa[/name], [name]Beatrice[/name], [name]Ophelia[/name], [name]Eleanor[/name], [name]Dominique[/name], [name]Cornelia[/name], [name]Alessandra[/name], [name]Valentina[/name], [name]Anastasia[/name], and [name]Natalia[/name] came to mind. [name]Hope[/name] this was what you were looking for!
I like [name]Octavia[/name]. It does mean “Eighth”, but [name]Thomas[/name] means twin, and is everybody named [name]Thomas[/name] a twin?
Other suggestions:
[name]Juliana[/name]
[name]Violetta[/name]
[name]Lavinia[/name]
[name]Carlotta[/name]
[name]Adelina[/name]
[name]Monica[/name]
[name]Seraphina[/name]
[name]Genevieve[/name]
[name]Felicity[/name]
[name]Bellamy[/name]
Iridessa (If you don’t mind repeating initials.)
Good luck! 
Names don’t have to be given based on meaning. Maybe [name]Octavia[/name] is used to honour a relative born in [name]October[/name] instead, if you want a meaning. Or the 8th of a month was the parents’ wedding anniversary.
Thanks! I like loads of these, but can you think of any good nicknames for these?
[name]Arabella[/name], that sounds quite posh and is the name of a posh character in the film A [name]Princess[/name] for [name]Christmas[/name]/[name]Christmas[/name] at Castlebury [name]Hall[/name]/A [name]Princess[/name] for Castlebury (it depends on what country you live in as to the name of the film)
Belladonna - [name]Bell[/name], [name]Ella[/name], Doni, [name]Donna[/name], Lady, [name]Bena[/name], [name]Adi[/name]
[name]Livinia[/name] - [name]Livy[/name], [name]Vi[/name], [name]Lina[/name], [name]Vina[/name], [name]Ivi[/name] (like [name]Evie[/name])