When I get caught in a massive siege, with many hundreds of attackers, it would be really nice to check on the number of enemies remaining to fight during the battle. Can this be done?
Yes, if you press the backspace key during battle it will pop up with what is variously called the command interface, tactical menu or backspace battle map.
What this looks like and what information it displays varies between mods but in my experience it always shows: 1) A minimap showing the locations of your troops, ally troops and enemy troops in different colours. 2) A table telling you how many of your own and your enemy's troops have been killed, knocked unconscious, routed and remain in the battle.
The tactical map is incredibly useful because you can click with your mouse to direct your selected troops to any location on the map. This means you can order your troops, particularly cavalry or skirmishers which you want to harass the enemy rather than engaging in prolonged melee combat, to move around without having to be there yourself. This is vital for infantry characters.
Here is a picture of the tactical map I pinched from Nexus Mods:
The tactical map only shows those enemies which have already spawned. If you're asking if there is a way to determine how many enemies are left to fight in the total battle then no there isn't. Check how many enemies have been killed/routed/knocked unconscious in the battle map and subtract it from either the total number of enemies in the battle (visible on the screen just before you choose to start the battle) or how many waves of how many enemies you have in your settings. Whichever of these is the smaller number will be the total number of enemies which will spawn in a battle.