we have an anviroment with more than 4000 items, we tryed to assign a template to over than 3000 Host.
Actually is impossibile for us to add a template to more than 1001 host.

consider that:

• hosts are imported from an xml file (is useful we can provide it no critical information inside)
• hosts are ALL assigend to a group (directly during import)
• we have tryed to assign the template directly from host panel, it seem to be assigned but after a few it disappear.. :-S

Disclaymer
i have try to search for similar issues but i can't found. sorry if duplicate

Max