SRB stands for signalling radio bearer and it is solely used for transmission of signalling messages such as RRC messages and NAS messages. Three signalling radio bearers have been defined in LTE . SRB0 which is used for transmission of RRC common channel messages. SRB1 which is established for UE and dedicated RRC messages are transmitted over this signaling radio bearer. SRB2, which is established to transmission of UE NAS signalling messages.
DRB stands for data radio bearer and a particular data radio bearer is established to meet a particular quality of service. For example: QCI "1" is established for data transmission of voice packets. Since Voice packets has its own delay budget and error rate.
As per my knowledge so far, an UE can establish maximum of 8 data radio bearers.
Out of these eight radio bearers, at least one bearer would be default and others will be dedicated. One default bearer represents one PDN connection and one PDN connection means one IP address.
Hope it will be helpful.