You mean to say simultaneously here: at a time how many data bearers can be possible. Please comment if it would not.
Data bearers means EPS bearers. We can have maximum 8 data bearers at a time because of logical channel id constraint. I know it is bit weird because in UMTS it was 11, we could have 11 PDP contexts.
But this is what in RRC_Configuration_Message we have one Logical Channel Id field which has maximum 8 integer values only. So we can not map more that 8 data bearers at a time.
In specification no. TS 36.508 Table 22.214.171.124-1 written that number of Data Bearers to be added (N) is 0<=N<=7
I hope it helps you.