Which word to use to describe the fact that something has increased by a factor of 8

Solution 1:

I have 10 rabbits and I want to write: The rabbits multiply eightfold each year.

Merriam-Webster

-fold suffix

Definition of -fold (Entry 5 of 5)

1 : multiplied by (a specified number) : times —in adjectives "[Profits showed] a sixfold increase" and adverbs "[I will] repay you tenfold."

2 : having (so many) parts "threefold aspect of the problem"

You say

Usually I would use the word octuplicate: The number of rabbits octuplicates each year.

Usually? Really? I can find no instance of "to octuplicate" as a verb. The OED includes it as a noun but also marks it as "rare" - It seems to be an adaptation of "duplicate". The example given is used humorously: "1994 Time 4 Apr. 88/1 Once a year, the government sends you a W-2 form, which you fill out in something like octuplicate."

Solution 2:

The verb is to octuple

verb: octuple; 3rd person present: octuples; past tense: octupled; past participle: octupled; gerund or present participle: octupling

make or become eight times as numerous or as large.

"the new town populations have trebled the additions to the rural community, and in the case of Massachusetts have octupled them"

https://www.lexico.com/en/definition/octuple

Usage has declined markedly in recent decades:

https://books.google.com/ngrams/graph?year_start=1800&year_end=2008&corpus=15&smoothing=7&case_insensitive=on&content=octuple&direct_url=t4%3B%2Coctuple%3B%2Cc0%3B%2Cs0%3B%3Boctuple%3B%2Cc0%3B%3BOctuple%3B%2Cc0%3B%3BOCTUPLE%3B%2Cc0#t4%3B%2Coctuple%3B%2Cc0%3B%2Cs0%3B%3Boctuple%3B%2Cc0%3B%3BOctuple%3B%2Cc0%3B%3BOCTUPLE%3B%2Cc0

Multiply eightfold would be more contemporary English, and more easily understood.