The FIFO rule for conversion distributions only applies between years, not to distributions of conversions done in the same year. So he can treat all 12 conversions as a combined single conversion. Of course, the total taxable amounts of these conversions comes out before any of the non taxable dollars.
This “aggregation rule” is stated with the Roth ordering rules in Pub 590 B.
NOTE: If there was a rollover from a 401k to the Roth IRA along with these conversions, that would also be combined into the mix as well.