The Windows 7 limit is based on the number of files (PCs) allowed in the Homegroup folder. This number is specifically noted as 2^32 -1 under NTFS. To answer your question, only one PC request is handled at a time since that is the limit in the packet handler of the communications channel. Technically, simultaneous access is impossible.

◦Maximum number of files on disk: 4,294,967,295
◦Maximum number of files in a single folder: 4,294,967,295