The match will be played on Saturday 6 April, at the home of English rugby.
But Ulster have said that they will be taking their full allocation of tickets and will be hoping to fill 25% of the stadium with travelling supporters.
Saracens chairman Nigel Wray revealed that the option of playing the match at their new home stadium, Allianz Park, had been considered.
But he added: "In consultation with local residents and our partners at the London Borough of Barnet, we have decided to settle at the new venue before applying to stage a one-off event with a larger capacity."
Wray also said his players were looking forward to playing at a ground familiar to many of them.
"It is absolutely clear that people want to see big teams playing big matches at big stadiums, and that is precisely what we will deliver at Twickenham," he said.
According to Ulster Rugby, ticket arrangements will be announced in the coming days.