To monitor the number of users imported into OneSync, you can set thresholds that will halt an import if the number of imported users exceeds that threshold number. There are four different types of thresholds, including Insert, Update, Enabled, or Disabled, to measure the number of users added, edited, re-added or removed from a source, respectively. If a threshold or thresholds are exceeded, a notification with the threshold type(s) appears next to the source in question on the source page, pauses the import, and prompts you to resume.
You can also be alerted to imports that exceed thresholds by adding your email to the System Email List in Settings. The emails in this list will be alerted every time an import is finished or a threshold is exceeded, along with the number of imports and their types.