Squad mates don't take away the place for any other possible squad mate. You have specific characters that are possible squad mates, you can't miss one due to accepting another squad mate.

The possible squad mates are:

James Vega, Liara T'Soni, Garrus, EDI, Ashley/Kaidan, Tali, and Vigil (a prothean from the DLC)

Squad mates don't take away the place for any other possible squad mate. You have specific characters that are possible squad mates, you can't miss one due to accepting another squad mate.

The possible squad mates are:

James Vega, Liara T'Soni, Garrus, EDI, Ashley/Kaidan, Tali, and Javik (a prothean from the DLC)