Luce Opus Magicum Fire Breathing

Hi,

What do the ‘n’s mean in the following paragraph?

It says: “ The ritual mentioned here is connected with the practice of breathing, which is executed according to the formula 2n (inspiration), n (retention), 2n (expiration), n (retention) in an early phase; and 2n, 4n, 2n, n in a later phase.”

I am familiar with the breath of fire in the kundalini tradition however I’m not clear on the context in the above paragraph.

Any clarification much appreciated.