Colm,
I've confirmed that's the limit (64 connections):
[pre]
%macro excelMany(howMany);
%do i = 1 %to &howMany;
libname test&i. excel path="c:\sas\data\test.xls";
%end;
%mend;
%excelMany(65);
[/pre]
This generates an error on the 65th assignment. The workaround is to clear the other libnames before you assign another one.
The MS Jet API (used for this type of connection) does have a limitation of 128 sessions, I've read. It might be that each libref connection uses 2 of those sessions in SAS, but that's a guess on my part.
Chris