The minimum is in AdbOptionsService.java:

  /**
   * Default user managed adb port. Same as {@link #USER_MANAGED_ADB_PORT_MIN_VALUE}.
   */
  public static final int USER_MANAGED_ADB_PORT_DEFAULT = 5038;

  /**
   * Default adb port +1 to avoid conflict.
   */
  static final int USER_MANAGED_ADB_PORT_MIN_VALUE = 5038;

  /**
   * Max ephemeral port number for most modern operating systems.
   */
  static final int USER_MANAGED_ADB_PORT_MAX_VALUE = 65535;

If the user has an adb server running on 5037 already, and they want Android Studio to use it, that is not possible because of this minimum. It appears to be an oversight as the comment describes the value as a default.

The minimum valid TCP port is 1.