Looking at the Birmingham - Newcastle corridor as the example I have timetables for, seats per hour changed from around 48 first, 390 standard pre-Operation Princess to about 52 first, 386 standard post-Operation Princess. Overall, no increase, but no decrease either, and the waters are muddied by the lengths of specific trains each hour. Other corridors would have been similar.
Prior to the 185s being introduced, Transpennine were operating 23x3-car (17 original and 6 hybrids) and 23x2-car class 158s. Assuming a one for one replacement by 185s, which isn't strictly accurate as there were some 4-car 158 diagrams operating on North Transpennine at that point, then that is in fact 7,774 fixed seats provided in both cases; but the 169 fixed seats of a single 185 is a fair step down from the 204 seats of a 3-car TPE class 158, or the 268 seats of a 4-car class 158 formation on any given train.