When setting a workflow's starting condition, you can choose to enroll contacts who already meet the starting conditions. Click the Estimate how many this would add link to see an estimate of how many contacts will be automatically enrolled in the workflow if this setting is turned on.
In some circumstances, just as with the Lists tool, this estimate will not be 100% accurate. For accounts that contain more than ten thousand contacts, the workflow's estimate tool samples 1/16th of your contacts to determine how many contacts meet the starting conditions. Therefore, as it pertains to such accounts, this estimate feature may not match the final count of contacts enrolled in the workflow, but will still provide you with an idea of how many contacts will match the rules in the workflow's starting condition.