Congress can't limit the # of laclerks..separation of powers, etc. Ehat would be interesting is to see more info on how the law clerks are selected..that is to say,who does the preliminary screenign of applicants for the justices...thee people control the ideological flow of clerk-candidates..
Technically, it's up to law students to seek out what Justices they want to work for and make an application. If they make the cut, they are interviewed.
In practice, however, there is a network of law professors at the prestige schools, better regarded appellate judges and SCOTUS (and their clerks) who identify and groom candidates. The talent pool is law review staff (top 5-10% of class) at the prestige schools. By self-selection that is a liberal group, although conservatives have made in roads. The Federalist Society is a helpful network for conservatives, and there are a number of conservatives who worked for conservative Justices. For example, Laura Ingraham clerked for Justice Thomas.
Basically, being selected as a SCOTUS clerk is being admitted to an elite, and like most American elites it's a combination of talent, achievement, networking and compatibility with the particular Justice. Blackmun became more liberal with every year on the Court, so it is not surprising he hired very liberal clerks.