When talking OSD (Operating System Deployment): we speak about "DS" (Altiris Deployment Solution)...
But when speak about DS, must ask for "6.9 or 7.1" ?
About DS 6.9:
We already build with success a single DS 6.9, with 60 remote sites (or perhaps 55 only, but 2 times). So we can afford 60 secondary PXEs, connected to a single central DS.
Where is the maximal limitation ?
Well, in theory, only a total of 5'000 machines can connect the central DS server. I am pretty sure we can upper this number using Windows 2008.
Regarding numbers of remote sites maximum: a secondary PXE will only connect the DS central a single TCP session...
We often got a problem using DS 6.9 because of any change preboot files, immediate impact thousand remote sites, and overload WAN. The workaround is to "stop" the PXE config helper on each site, to start only during "maintenance hours" (...