It is however very easy to do it yourself and adjust the cap to your preferences; be aware however it will also be changed for the AI.
Before we start you need to download the Total War Pack File Manager here:
http://sourceforge.net/projects/packfilemanager/
1) Open it and go to file>My Mods>New on the top left corner
2) Select a name for your mod
3) Select your "steam\steamapps\common\Total War Attila\data" directory
4) Select the "steam\steamapps\common\Total War Attila\data\data.pack" file
5) Go to the data.pack>db>fame_levels_tables>fame_levels location and click ok
6) Go to the "Your mod name">db>fame_levels_tables>fame_levels location
7) The second column (named "unknown1") is the number of armies, in the eleventh column ("unknown10") you see the associated imperium level and the tenth column ("unknown9") shows if its for the main campaign ("main_attila") or the prologue ("pro_attila").
I do not know why there are two entries for level 7 so I just changed both values for those to the same number.
Also be aware the other columns can be used to change the number of navies, agents and governors. I did not try which one does what yet however so you might need to experiment.
8) Make sure your created mod.pack is in your data directory and launch the game. Activate the mod in the mod manager (activate out-of-date mods)
9) Start a campaign and make sure your changes have worked out
I have attached a link to the version I have done for my ERE campaign, you can also use it as a base to adjust to your own preferences if you like:
http://www.gamefront.com/files/24864119/Army_limit.zip
It only changes the amount of armies from level 4 upwards:
4 -> 12 armies
5 -> 14 armies
6 -> 20 armies
7 -> 26 armies