When setting a workflow's enrollment triggers, you can choose to enroll contacts who already meet the enrollment triggers you've selected. Click the Estimate number of contacts link to see an estimate of how many contacts match your workflow's enrollment triggers.
In some circumstances, just as with the lists tool, this estimate will not be 100% accurate. For accounts that contain more than 10,000 contacts, the workflow's estimate tool samples 1/16th of your contacts to determine how many contacts meet the enrollment triggers. 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 enrollement triggers.