Around 20,000 Rangers fans will be in the Estadio Ramón Sánchez-Pizjuán to cheer on their side in the Europa League final.
Club bosses believe half the fans in the 40,000 seater stadium will be Light Blues fans - after diehards more than doubled the number of tickets handed to the club.
Only 9,500 tickets were given to the Ibrox side for the showpiece clash with Eintracht Frankfurt by UEFA.
But the club believes supporters snapped up an extra 10,000 tickets, getting their hands on briefs by any way they can.
As well as buying from fans of other teams whose clubs never made the final, Light Blues supporters have snapped up tickets from tout websites, the first UEFA ballot, and a second release by the body.
It's understood the second ballot, which was released at the weekend, was a batch of tickets which were rejected by sponsors, such as Heineken and Hankook.
Those tickets were offered to fans who'd be unsuccessful in the first ballot.
A club insider said: "There were 9,500 given to us, which have all been allocated as per the club's policy.
"We anticipate there being just over 40,000 being inside the stadium, as fans are picking up some of those tickets from Heineken, Hankook etc.
"I think we will probably pick up anything between 16,000 and 18,000 tickets."
And fans without tickets have Seville Mayor Antonio Muñoz to thank for there being safe places in the city for them to be able to watch the game.
The insider added: "Five members of staff went out last week, spending three days in Seville.
"They worked with UEFA and the authorities in Seville to make sure it's as safe and welcoming for Rangers fans as possible.
"We worked very hard and managed to secure a stadium, in the form of the Estadio La Cartuja, and numbers continue to grow for that.
"We are working to ensure screens are working and that there are adequate refreshments, food and water for fans, and that's in a controlled environment.
"The club has built up a good relationship with the Mayor.
"Initially the police would not allow the use of the Cartuja stadium.
"But the Mayor was the main individual who pushed that forward, politicians have pushed this and made this happen."
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