In her evidence to the inquiry, Mrs Brooks said she had exchanged messages with Mr Cameron about once a week while he was opposition leader - though that increased to about two a week in the run-up to the 2010 general election.
Most messages, she said, were to discuss "organisation" or future social occasions.
She said he would mainly sign off the texts "DC" but would occasionally sign them LOL, meaning lots of love until she told him it meant laugh out loud.
Mrs Brooks has been arrested over the phone-hacking scandal and charged with three counts of conspiracy to pervert the course of justice, which she denies.
Her husband Charlie Brooks - who went to school with Mr Cameron - has been charged with one count of the offence.
Mr Coulson has also been arrested over alleged phone hacking and has been charged with perjury in a case connected to the scandal.
He resigned as Mr Cameron's director of communications in January 2011, blaming coverage of the NoW phone-hacking scandal.
The prime minister has already submitted his witness statement and evidence of key meetings, texts and emails.
He will be the sole witness on Thursday.