This doesn't mean the U.S. will seek the death penalty --- that decision is probably months away. Under federal law, every defendant charged with a crime that carries the potential for the death penalty is entitled to two lawyers, at least one of whom must be "learned" in capital cases.
§ 3005. Counsel and witnesses in capital cases
Whoever is indicted for treason or other capital crime shall be allowed to make his full defense by counsel; and the court before which the defendant is to be tried, or a judge thereof, shall promptly, upon the defendant's request, assign 2 such counsel, of whom at least 1 shall be learned in the law applicable to capital cases, and who shall have free access to the accused at all reasonable hours. In assigning counsel under this section, the court shall consider the recommendation of the Federal Public Defender organization, or, if no such organization exists in the district, of the Administrative Office of the United States Courts.
If indigent, learned counsel will be appointed upon being charged. It's not necessary to wait until the Government files its notice of intent to seek the death penalty. Here's a case from the 1st Circuit (which includes Mass.):
In this case, we are asked to construe language in 18 U.S.C. § 3005 (2000), providing that following the indictment of the defendant on a capital crime, the court "shall promptly, upon the defendant's request" assign two counsel "of whom at least 1 shall be learned in the law applicable to capital cases...." We conclude that "promptly" means promptly after indictment, not (as the government asserts) only after the Attorney General has made a determination to seek the death penalty.
There is no set limit on the number of attorneys that can be appointed to represent a defendant in a capital case.
In related news, Police say Dzhokhar Tsarnaev indicated to authorities the brothers acted alone. They believe the motive was "religion."