World football's governing body had wanted the 12 venues to be ready by December last year, but only the six used in the Confederations Cup last June were ready by then.
Of the remaining six, three are still not complete with the opening match set to take place on June 12.
"We can't say exactly how many tickets there will be because we don't know the exact number of seats in the stadiums," Weil said at a news conference in Rio.
"We have held back 7 percent of the tickets until we really know how many tickets will be available."
Brazil has had seven years to prepare for the tournament but is still racing to finish stadiums, airports, roads and other vital infrastructure.
At least one airport will greet passengers in a tarpaulin terminal building and several cities have either cut back plans to build roads, bus lines and railways or shelved the original plans altogether.
The biggest concern for FIFA is the stadiums and particularly the Arena Corinthians, which is scheduled to host the opening match between Brazil and Croatia on June 12.
The stadium was built to hold 48,000 people but some 20,000 temporary seats are being added for the tournament's six games and they are still under construction.
Stadiums in Curitiba and Cuiaba are also unfinished. Some 27,000 seats are still to be installed in Curitiba.
Weil said he hoped to have a definitive figure by the middle of May and begin last-minute sales soon after.
"At some point in May more tickets will go on sale with the completion of the stadiums," he said. "It's just taking time."
Another fear for FIFA is that it will not have enough of a window to test the stadiums properly.
"We need to test everything, the communications, lights, cables, all the things that need to work and unfortunately we can't do that in these stadiums," he said.
"We'd love to test these kind of things and then have time to react to any problems but we'll have to accelerate the process."
FIFA have so far sold more than 2.7 million tickets for the 64 matches, 58 percent of them to fans in the home nation. There are 200,000 currently on sale to the general public waiting to be snapped up, he said.
Of the more than 3 million tickets that will be available, 1.7 million were bought by fans through the FIFA ticketing web site, an increase of 400,000 on the previous record number sold that way in Germany in 2006.
- Sports & Recreation