If you are in California, you need a minimum of 60 semester units to transfer as a junior to a UC, but you can take more than that at community college before transfer. After transfer, the transfer credit for community college units will be capped at 70 semester units. However, all courses completed at community college count for subject credit to fulfill subject requirements (e.g. math, physics, general education, etc.).
Credit unit limits for transfer eligibility to UC come into effect if you have upper division credit from four year schools, or any credit earned at UC.
http://admission.universityofcalifornia.edu/counselors/transfer/advising/transferring-credits/index.html
http://www.sdmesa.edu/student-services/transfer-center/documents/uc-transfer-maximum-limitation-policy-chart.pdf