I recently got a hold of a bigger version of this “monster concert” held by Jullien in London. As you can see from the details, there are more trombones than just the 4 that are easiest to see in the top of the first section. Farther up, in the full row below the huge bass drum are 3 (possibly 4) more. I’ve added the 2 details to the full image in the Trombone History Timeline (19th century, first half).
1846—London, England: An image printed in the London Daily News depicts a “monster concert” staged by Louis Antoine Jullien at Covent Garden (see 2 details and full image below). The performing forces, comprised of Jullien’s orchestra and 4 military bands, include at least 7 trombones: 4 at the top of the first section of orchestra seating (see top detail, below), and 3–or possibly 4–on the third row of the steeper risers (see second detail, below) (public domain) (Remnant, Musical Instruments of the West 218).