The post you are looking for is a sticky in the Timing and Scoring area of the forums,
Driver Numbers is the title.
You can not "reserve a number" you can only register with the number that you want and if you are the first person to request a # that isn't previously reserved, then that will be your # for the entire year. You can only get a # reserved for you if you are an OMR SCCA member that ran in at least 2 events the prior year.
Right now 21 is not a reserved number (ironically it was abandoned by a SM competitor for a different #) so if you are the first one to sign up with that # for the May 16th event, when we open registration, you will receive that # for the rest of the year. If you run at least 2 events with us and are an OMR SCCA member then you are guaranteed that # next year.
I suggest to all new members/participants to use blue "painters tape" (preferably 3M

) their first time out just to make sure that they have the number that they want, and if you order #'s make sure to get an extra in case you ever run an event with a different club you will be able to switch it up in case your number is already being used in that event.
Hope this helps, and we are looking forward to seeing you out there.