Same as any PC - 15
A long time ago some Geeky post-billionnaire thought 'Who will need to connect more than 15 things to a PC... and it's stuck. Things like IRQ Steering make life easier for us, but regardless of CPU - The system architecture of the PC dictates 15 IRQ's and some are reserved like IRQ2 for (IIRC) system clock etc.
IRQ's are best ignored unless you're getting conflicts.
Signatures reduce bandwidth