CRS shows the specific classes' available slots and demand garnered from students. Moreover, you are more likely to be granted a class if the available slots are greater compared to the demand. However, you can always try your luck if you want a slot in one with bigger competition.

Take note of the following! The enlistment process does not follow a "first come, first serve" policy. Moreover, how you rank your classes does not guarantee a higher chance of obtaining your top choice. Be strategic! Take advantage of your freshie priority, and maximize the 20-class limit to secure your desired courses.

Pick classes that align well with your learning style and schedule! SIK suggestions and ask your friends and batchmates for any professor recommendations. Gauge your class expectations by familiarizing yourself with your chosen professor's teaching style and requirements.

Choose a class time that best works for you! If you're a morning person, maybe the morning classes suit you best. Else, SIK the later or afternoon schedule of your desired courses.

There will be a chance you won't get your desired number of units, and that's okay! Once the first round of pre-enlistment finishes, you will still get an opportunity to choose subjects in the following batches of enlistment. Moreover, the newly added courses with an updated number of available slots and demands allow students to enlist in their desired classes. 

If all hope is lost, SIK the blessing of the CRS gods to increase your luck. These may or may not help you, but it surely won't hurt if you try. Share the beloved CRS Pepe and other memes to manifest full units. May the odds be ever in your favor!