The leap in sign-ups to 1,000 comes only a month after Brill, who launched Journalism Online in April with partners Gordon Crovitz and Leo Hinder, announced 500 had signed letters of intent.
The news organisations come from the across the world and include Guardian Media Group in the UK and the Milwaukee Journal-Sentinel in the US among others.
Separately, News Corporation and MediaNews Group have said they also planned to charge.
A key factor in the numbers signing letters of intent, according to Crovitz, is that publishers have concluded that this is not an either/or choice between advertising revenue and circulation revenue.
Brill said: "Publishers are focused on the revenue potential of a mix of free and paid access to their sites, with the 'freemium' approach increasingly popular."
The news from Journalism Online comes as a survey from the American Press Institute reveals that 58% of newspaper publishers believe readers will pay for content and are actively looking at a timetable.
The API survey found that 51% of publishers believe they can make a success out of paid content while 49% are less convinced.
The study, which spoke to 118 US newspapers, also highlighted the variety of approaches that publishers are considering for paid content with 38% predicting limited access beyond the homepage to full stories, to those who don't buy a monthly subscription.
28% are considering monthly subs and micropayments while 15% were looking at monthly subs, micropayments and day passes.
Others (19%), said they thought news would be free, but would charge for specially produced premium content and 9% said they were considering a pay-as-you-go story by system.
Brill said that the results of the API study were "absolutely consistent with what we are finding in the United States and around the world.
"That flexibility and diversity is also consistent with our view, which is why our Reader Revenue Platform has 16 different options for publishers."