Edit - yes, take Prisma's advice. Unless you want to learn the XML and get into modding, use someone else's modlet. I'm a glutton for punishment so I DIY some of this easier stuff.
Original post below.
Sure. If you want to directly edit spawning.xml, then look for this section:
Code:
<biome name="pine_forest">
<spawn maxcount="1" respawndelay="3" time="Day" entitygroup="ZombiesAll" />
<spawn maxcount="1" respawndelay="4" time="Night" entitygroup="ZombiesNight" />
<spawn maxcount="1" respawndelay="14" time="Any" entitygroup="EnemyAnimalsForest" spawnDeadChance="0" />
<spawn maxcount="1" respawndelay="21" time="Any" entitygroup="FriendlyAnimalsForest" spawnDeadChance="0" />
</biome>
Simply change "maxcount" to however many zombies you want in each spawning area
* and change "respawndelay" downward if you want them to respawn more quickly (in game days). You can change day and night separately and change the hostile/friendly animals too. I upped them all and reduced all of their respawndelays as well (doing this for friendly animals will result in much more raw meat availability, so balance to your playstyle preferences).
*I am not certain how big a "spawning area" is, but it seems to have maybe a 5-10 chunk radius? Assume it's 10 chunks, so roughly 320x320m area. Setting maxcount=4 means at most 4 zombies will spawn in that area. As you drive through a very large forest you will go through many of these areas and each of them can have at most 4 zombies (they can wander into other areas of course). So it's not "4 zombies worldwide in forests"...you will see dozens of zombies as you move through the biome.
I have mine set to maxcount=4 and respawndelay=1 at the moment and it sure feels more like the old console game now. Push it as far as your PC (and you) can handle.
Repeat the above for the next few biomes - wasteland, desert, burned_forest, and snow. If you want a little variety in your snow biome, add this line in there anywhere between <biome name="snow"> and </biome>:
Code:
<spawn maxcount="2" respawndelay="1" time="Day" entitygroup="ZombiesAll" />
If you want to do it as a modlet, I can help you via direct message if you want (be warned - I am a modding neophyte; I just know how to use XPath, none of the fancy stuff).