They don’t need all 53. That’s partially why this is entirely theater meant to damage Trump and hurt GOP senators in purple districts. There’s no way the prosecution will get 67 votes to convict.
Right, the defense only NEEDS 34 votes to acquit, but they want far more than that, in order to get total vindication for Trump.
The defense team wants all 53 Republicans and a handful of Democrats to vote against conviction/removal, when that is finally voted on. Support of all 53 Republicans sends an important message to the public: that this entire impeachment was never anything but a political attack by Democrats on Republican President.
The defense will gladly make concessions on procedural matters, hear more witnesses and let the trial proceed a bit longer, if doing so gets those stray Republicans back in line.