When you setup a mentoring program you will have two options for the setting "Pairing Approval Type" - batch and rolling. Changing this later can cause issues, so it's worth taking a minute to think through which is best for you before turning mentoring on.
In most cases, "rolling" is likely the best choice for your program. With "rolling", registrants will be able to be paired immediately after they register. Moreover, every user's mentorship cycle can start when they are paired (i.e. be on a different cycle than the other relationships in the program). Using rolling also allows you to add a mentor and/or mentee approval step to the process so that mentors and mentees have the opportunity to review a pairing before it is finalized.
With "batch", all mentorship relationships occur on the same cycle. That means all users receive their pairing assignment email at the same time and are asked to complete their sessions by the same date time. While this helps limit the amount of email traffic by consolidating all pairing notifications, it does pose some limitations. First, as mentioned above, you cannot have a mentor and/or mentee approval step under the batch process. Second, you cannot make changes to pairings or have new registrants mid cycle. Any changes need to wait until the start of the next cycle.