That's not an extreme number of frequencies or talkgroups. That scanner can hold 1,800 active "objects," which are conventional frequencies (e.g., 155.3400 FM) or talkgroups (e.g., 30201 Cobb County Fire Dept. Dispatch).
For a trunked system, all of the frequencies for a site (Cobb County) must be programmed into the scanner. Newer scanners, like the -106, will allow you to program only the control channels (but plug-in the blue and red ones). This reduces the number of frequencies that have to be entered.
You can split up your listening on that scanner in a number of ways:
1. All of Cobb County (every agency) in one scanlist
2. Only Cobb County Fire Dept in one scanlist
3. All fire departments in one scanlist (including the other counties you listed)
4. All of the above, so you can change your listening on-the-fly.
Etc.
If you are hand programming any scanner, that may seem like an impossible mission. I strongly recommend that you buy the appropriate scanner programming software. It's well worth the cost. I paid for WIN500 for my PSR-500 and it's been worth every dollar, year after year. Here's a list of good programs to check out (and buy):
Object Oriented Scanner Software - The RadioReference Wiki
You have this bookmarked, right? >>>>
Easier to Read Pro-106/197/PSR500/600 Digital Scanner Manual