I've found the number of enemies is a bit unbalanced compared to my character. Can I increase their number a bit?
There is currently a mod out, PISE -Improved Skyrim Experience which at least attempts to spawn more and harder enemies:
Generally, the spawns for creatures should, on average, be increased by 50%. Level caps are raised 6-8 levels above normal for a certain list.
However, Skyrim uses multiple techniques to spawn enemies and some of them are not amenable to simple spawn count increase via modifications to the leveled lists, so this mod doesn't work in all cases.
If you are playing on PC, there is a much simpler, no-download way to do this.
Press the tilde key (~) on your keyboard while in-game and increase your level with the command:
This will increase your level by 1. You can stack them by typing:
The following is an excerpt from UESP:
...you are not given the option to increase an attribute and you are not awarded any perk points, nor, obviously will not have any of your skills improved... so if you're a Tamriel veteran and have already cranked the difficulty to "Master" but still crave more of a challenge, using advlevel to add ~10 levels to your character should certainly make the enemies more difficult.
EDIT: As Raven Dreamer said, this will only apply to leveled enemies.
this will require the construction kit, after which, if it is anything like MW/OB, it will then need a relatively simple but tedious job of adjusting the levelled lists.
Alternatively, more elegantly and more powerfully, the new system of adding things through script / quests that will be used to add new items and NPCs will probably allow a nice script/quest to add additional enemies rather than making hard changes to the levelled lists.
TL;DR: Not much can be done until January.
EDIT: I was wrong, see post above for a somewhat working method :)
This mod spawns random enemies if you didn't find any foe in the last 40 seconds: Enemy Encounter