Use http://www.assist.org to help you select courses that cover the prerequisites for your major and other requirements (general education or IGETC) at each target UC and CSU. You do not need huge numbers of units (and huge numbers of units may be detrimental if you have any upper division course units), but you should make sure to cover any course requirements that you can.
Regarding UC Extension courses, check the course numbers, as described at https://extension.berkeley.edu/static/studentservices/transcripts-details/#prefixes . The courses prefixed with just X may not transfer or be given subject credit; those prefixed by XB are considered equivalent to regular UCB courses.