A question about the resource mapping according to TBS table

However, I still encounter some understanding problems about calculate the resource mapping according to TBS table for uplink.

For example, let’s assume that we have got an setting of MCS with index I_TBS = 1 and I_RU=0. Which means we use 1 resource unit to construct TBS with 24bits and modulation order is 2 (means QPSK)according to TABLE 36.213- The question is, what’s the eactly subcarrier indication field I_SC would result in such a TBS?

According to definitions, there are four methods of subcarrier allocation for 15kHz. And 7 symbols per slot. Given any combination of possible subcarrier allocation method, e.g., 1x16, 3x8, 6x4, 12x2 (# of subcarriers x slots), I can not construct the TBS 24 bits. But a relatively large block size, for instance, 12x2x7x2 = 226 bits (# of subcarriers x slots x symbols per slot x QPSK bit

Could you plz give me some help? Thank you very much!