Your host is going to run you off way before you start nearing 50k files. 200gB/2tB accounts are the equalizent of a server. If you're not paying at least $150 for that account, chances are you're with a host that's overselling their boxes and will run off anyone coming near those limits, usually with an excuse that it's your fault and you're overmaxing their box. Please remember "If it sounds too good to be true, it probably is."
To answer your question though, it really depends on if you're going to open up your site to the public and how much space you're going to allow for uploads. I think for most of us here, only about 5% of signups actually stay around longer than the first month.
You start to run into problems near the 1.5k mark of blogs. By then, your database will then have 36k files within it if you're not removing sblogs. That's probably where the limit will be reached first if they also count the files that make up the database. You'll probably want to ask them that.
Do also note that changes are reselling or giving out subaccounts is usually against most hoster's standard account ToS'es. I would also double check that. If you;re going with a cheap host like you appear to be doing, I would be alomst 100% sure that it is.
Hope this helps,
-drmike