Audacity can record from one device at a time.
Audacity can record as many channels as that device offers at one time.
If a recording interface offers a single device with 8 recording channels, then Audacity can record up to 8 channels at a time. If you select less than 8 channels, then Audacity will record the number of channels selected, starting at channel 1 and counting up. Audacity does not have “channel mapping”, so you cannot, for example, record channels 5 & 6 from an 8 channel recording device without also recording channels 1, 2, 3 and 4.
If a recording interface offers 8 channels as 4 stereo devices, then Audacity can only record from one device at a time. So if the 4 stereo pairs represent inputs 1 & 2, 3 & 4, 5 & 6, and 7 & 8, then you can record from any one of the pairs.
The number of channels and devices depends on both the hardware and the device drivers.
Audacity is not able to directly record from 2 devices at the same time, though it may be possible to create a “virtual aggregate device”. This is when you create a “virtual” (software emulated) device that takes multiple real devices as its inputs, and one multi-channel device as its output. This ability is provided by Mac and Linux but not Windows.
On Windows it may be possible to create a virtual aggregate device using third party software such as “Virtual Audio Cable” (VAC https://en.wikipedia.org/wiki/Virtual_Audio_Cable). There is a free trial version of VAC available from their website.
M-Audio Delta devices can usually record multi-channel into Audacity using the older (XP) drivers hen selecting the “multi” device in Audacity. Obviously using XP drivers on Windows 7 is not recommended and may not work. There is a small chance that changing host in Device Toolbar to Windows WASAPI may let you choose more than two channels at a time, though this only works for a few sound devices.
Compiling Audacity yourself with ASIO support would almost certainly let you record multiple channels from Delta using the correct drivers.