This is a very workable solution. There is no built in limit and you can look over the system logs to see if any machines take too long to back up due to slowness and lighten the load on any one Time Capsule as needed.
For some people it's ideal, very maintainable, and drop dead simple to plan and support.
You can use BackupLoupe to look into the individual backups to see what files are changing and estimate how fast each Mac is filling up the space on the backup drives. You could use one Mac connected to the Ethernet port to scan the files without any WiFi delays and once scanned, you have the information on that Mac to examine "offline". You can mount the volumes and scan them as needed to update them as time passes - maybe once a month to check on things.
Do run a comparison on how much it would cost to buy a Mac mini server and use it to back up everything in one spot instead of several Time Capsules. You could then save money on the networking with Extremes and Expresses and have better expansion options. I would guess this is more cost-effective and easier/more powerful if you are buying three Time Capsules and might even be better even if you only are buying two Time Capsules.
Setting aside the cost of the initial hardware, a back of the napkin comparison might look like this assuming you need the TC for WiFi networking as well as backup functionality:
Three Time Capsules
- Easier to set up initially (if you guess well or over-buy storage you may never need to maintain which Mac backs up where minimizing maintenance)
- Takes more time to plan and track which Mac backs up where
- Harder to predict when space
will run out
- Add space in increment of new Time Capsules (pricier storage)
- Takes more time to track / reconfigure clients when adding more space (especially if you intend to move the previous backup to the new TC and don't want to cut that user off from their backup history)
- TC backups are harder to backup, archive, or move around (if you even need this)
One Mac mini + Airport Extreme + optional AirPort Express to extend the network
- Harder to set up initially (maybe one day's reading, learning, and doing)
- No hassle or time to shuffle or track which Mac backs up where
- Easier to predict when space will run out
- Can add more space by adding more drives (less pricey storage)
- No need to reconfigure clients as more space is added
- Very easy backup of the server and backup data (if you even need this)
You can minimize the work on maintaining a fleet of Time Capsules by periodically backing up each Mac to a connected HD and then deleting the backup for that Mac and starting Time Machine fresh. Rolling those with 1/4 of the Macs every three months will spread the work and let each Mac generally have a year of history before it gets erased and starts anew.