You can connect as many (supported) trackers as you like. Count.It looks at the activity data from all your devices/apps, and, by default, credits you with the best score for any given activity from whichever source has it.
For example, if you use two apps to track your steps, Count.It will take the score from whichever one shows the most steps for a given day — and ignore the other. If you want to know which data sources have reported activity for a given day, simply go to your Count.It Logs page, and click on the progress bar for that day:
If, for whatever reason, you want to change the "active" data source for a day, you can override the default setting by selecting the radio button to the left of the data source. When done, click "Save." This will NOT change the default setting for any other day.