There is not a limit on the number of emails you can store in a .PST however there is a size limit as well as a practical size limit. For starters Outlook 03/07 can have a maximum of 20 gigs in a PST and Outlook 2010 allow up to 50 gigs, source. I can not stress enough that for Outlook 03/07 you not let your PST go over 2 gigs. It may work fine for some people but in my experience the majority will see major issues over time, particularly if you are moving it from computer to computer. The problems are a mixture too, sometimes outlook will fail to start, sometime it’ll stop sending and receiving, sometimes it will randomly delete mail, other times the entire file can get corrupted. The best part is all of this is entirely random. We went through hell at my company before implementing email archive because users would consistently build pst that were 2-10 gigs with their archived mail. I would definitely suggest breaking it into multiple PST's, for example 06-07 and 08-09 instead of one giant pst. Of course if you are lucky enough to fit 80,000 emails in under 2 gigs you should be fine, but unless they are mostly plain text I can't see this happening.