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Also, not all UEs need to be multiplexed onto one PUCCH. For example, for 6 RB configuration, maximum 6 UEs can be given DL allocations which mean only 6 HARQ-ACKs are needed in UL. For higher bandwidth configurations (15 RB and above), more number of HARQ-ACKs will be required to be sent, in that case more number of PUCCH resources can be configured (i.e m > 1).
In addition, SRs are irregular (also need not be instantaneous) and possible to combine with HARQ-ACK by intelligent use of modulation technique. Similar is the case with CQI and HARQ-ACKs. More details on this at [36.211:5.4].
Essentially, by carefully organising PHY UL signaling, and using orthogonal sequences and modulation techniques, multiple UEs and different UL signaling can be multiplxed onto shared PUCCH resources.
Note that above PUCCH signaling is applicable to case when UE is not using PUSCH in same subframe, in which case UCI is sent on PUSCH.
Details about BSR and SR are available in MAC specification 36.321, sections 5.4.5 and 5.4.4 respectively. PUCCH organisation, PHY processing of UL signaling along with UCI formats is available in 36.211, section 5.4. This one is to be read along with section 10 of 36.213 "PHY procedures".
You may as well refer chapter 17.3 of 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming and chapter 17.3 of LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker.
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