You may see that there are X number of profiles in your main list or SMS list. However, when trying to send a SMS campaign, you may see that this campaign is expected to only send to a portion of this list.
Most likely what is happening is that you do not have SMS consent added for these profiles. For your account to send SMS messages to the profiles, you MUST have SMS consent added to the phone number. It could also be that the profiles were consented to receive SMS at the time when the Campaign was scheduled but opted out recently and thus are no longer eligible to receive SMS campaigns.
This is why you are seeing that only a portion of the profiles from the list/segment can receive SMS campaign messages. I would highly suggest going over this great guide on Understanding explicit vs. implicit consent and gathering SMS subscribers based on Guide to collecting SMS consent.
To see everyone who is currently consented to receive SMS, you can create a segment following our How to create a segment of SMS subscribers guide.
I hope this enables you to achieve your goals. You can also connect with your peers, explore more resources, and ask questions in our Community. Otherwise, check out these resources on how to send an SMS campaign: