Find the number that fits somewhere into the middle of the series

Find the number that fits somewhere into the middle of the series. Some of the items involve both numbers and letters Look at this series: VI, 10, V, 11, __, 12, III, ... What number should fill the blank?

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #59636