If the number of selectable contacts shown in the SMS Blast app is lower than your Wix Contacts count, this is likely because you don't have phone numbers added for those contacts.

To check this, scroll down below in the table of contacts. You should see that not all of your contacts have phone numbers. (For example: If the number of selectable contacts is 290, then starting with the 291th contact, there will not have a phone number displayed.)