Wow, a Seminex question! You never know what's going to show up on this sub! LCMS history is a relatively obscure branch, but yes, I can help you with this one!

You clearly know at least part of the back story, but for other readers... Concordia Seminary in St. Louis, MO is one of two denominational seminaries of the Lutheran Church - Missouri Synod (LCMS), one of the major Lutheran church bodies in America. On February 19, 1974, as the OP says, many of the professors and students staged a walkout in protest of events within the synod. Broadly, there was a conflict between the kinds of academic methods then used in mainstream secular scholarship, like historical criticism, and a more conservative, fideistic approach. This had been building since the late 1940s and really started heating up at the LCMS national convention in 1969. It was also exacerbated by the personality conflict between synod president JAO Preus (not coincidentally the former president of the other LCMS seminary, then in Springfield, IL and now in Fort Wayne, IN) and CSL president John Tietjen.

So to your question, how did quickly CSL recover from this walkout? A few numbers: the early 1970s were the high water mark for CSL enrollment and LCMS membership as a whole. There's demographic reasons for this beyond the effects of Seminex, but the 1973 CSL graduating class was the largest in the seminary's history to date. I can't lay my hands on the exact number, but I believe it was approximately 165 graduates. The total enrollment of the school in the 1973-74 academic year was around 600. A little over 100 of those remained for the Spring quarter of 1974, after the walkout, so it was more than 200 students who left - about 85% of the students left, along with 90% of the faculty. As for the professors, there were 45 before the walkout; of those, only five remained.

Only 37 students graduated in the spring of 1974, scarcely 1/5 of the previous year. In the fall of 1974, the beginning of the next academic year after the walkout, total enrollment was 193 - barely more than a single class in the recent past. Yet the recovery was surprisingly quick - the next year, enrollment was 284, and in 1976 it was up to 365. Three years later, the 1977 faculty was up to 25 - enough to effectively operate, certainly, but not to the level it was before.

Numbers alone can't tell the story, but they help point us to the perception of the situation by the wider LCMS membership: the perceived problem was not so much with the CSL institution per se, but with the specific faculty members and what they were understood to be teaching. When those faculty and students left, it was a blow to the seminary, but it wasn't enough to dissuade all prospective students from attending. CSL was considered the flagship seminary of the synod, and the more academically rigorous, "theoretical" school, while the Springfield (now Fort Wayne) seminary was the "practical" school. Even with the significant blow to its reputation that Seminex caused, CSL was still, in many people's minds, the main school. Even today, while the theoretical/practical distinction has faded, Fort Wayne enrollments are markedly smaller than CSL.

Part of it was also perhaps pragmatic. Though the walkout faculty continued to operate their own seminary for a few years (Concordia Seminary in Exile or Seminex), they were severely hampered by the lack of pastoral positions open to their graduates. Most LCMS congregations didn't want a Seminex graduate, and other Lutheran church bodies were also slow to open up to them. Yet, there was certainly an amount of sympathy for the Seminex position in the synod - so for a prospective student, the Springfield option could be unpalatable because of that school's perceived lack of academic rigor and conservatism, while Seminex was unwise due to real lack of career prospects; that left CSL, even in its current state, as the best middle ground. The LCMS did set up a process by which a Seminex graduate could be colloquized into the LCMS, and about 25 of the 1974 Seminex class did end up becoming LCMS pastors, but overall colloquies were fairly few. In addition, some of the LCMS district presidents remained sympathetic to the Seminex situation, and some of them defied the national synod to place and ordain Seminex graduates anyway. For doing this, eight district presidents were officially reprimanded by the synod and resigned or were removed from office. So it quickly became clear to most young prospective LCMS pastors that, their own sympathies notwithstanding, if they wanted to remained in the LCMS they had to choose between Springfield and CSL.

As for the blow to the LCMS as a whole, in the end about 250 congregations left the synod, out of around 6500 total - so a loss of ~4%, not a crippling blow. Even outside observers (articles in the St. Louis Globe-Democrat and Time magazine) noted in the fall of 1974 that the school was doing surprisingly well in the wake of the walkout.

Sources: Exodus from Concordia, a report from the CSL Board of Control on the walkout, is an excellent source. The perspective is, of course, quite slanted, but the basic narrative of events and the numbers it provides seem accurate. For the Seminex side of the story, see John Tietjen's Memoirs in Exile or the faculty's Faithful to our Calling, Faithful to our Lord. Other sources include Paul Zimmerman's A Seminary in Crisis and James Burkee's Power, Politics, and the Missouri Synod (though I see you say you've already got that one).