I use 7x MICCA in-wall speakers (M-8s and M-8c). Each one is 100W. With the subwoofer, I get great results with this 5.1.2 configuration.

Denon's official power specifications should be interpreted with caution: Of the stated 180 W/channel (1 channel at 6 Ω) or 135 W + 135 W (stereo operation at 6 Ω), 80 W/channel with 6 Ω remain in 7.1 operation (Audiovision 12/22 test). I don't know what happens with 9 active channels. However, due to the increasingly compact design, the device gets very hot. I had the AVR-4311 as a predecessor model, according to Denon 170 W + 170 W (stereo mode with 6 Ω), in the Audiovision test just 62 W/channel was measured in 7.1 mode with 6 Ω. Even in 5.1 mode, I had massive overheating problems with repetitive emergency shutdown and heavy clipping in the summer despite a well-ventilated setup.
I've been running the AVC-X3800H in a 6.1.4 setup with 6x B&W DM 603 (max. 100 W/8 Ω) and 4x Klipsch R-50M (75W/8 Ω, peak 300W) since 05/24. To avoid the above problems, I purchased the IOTAVX AVXP1 7-channel power amplifier right at the beginning (in 7-channel mode 120W at 8 Ω or 170 W at 4 Ω) with an excellent value for money (currently available in Germany for EUR 1379.00, e.g. at grobi.tv). The floor speakers are handled by the IOTA, with the ceiling speakers alone the Denon is under little load, only gets lukewarm and the sound quality is excellent.

Yes, you need a second amplifier if you want to run 11 channels. I took an old Denon 520ae and connected the front Li+Re to take the load off the 3800

I'm not a professional either, but this AVR has an output of 9 x 180 W. I operate it as a 5.1, but could easily connect another 4 speakers (e.g. for Atmos). If I understand correctly, you want to run a 7.1.4. Or do you have that and want 2 additional speakers? You may then have to buy an 11.1 AVR.