Data usage limit per line on the account?

Sorry but no, there is no way to do that. The best you can do is monitor each phone's data usage with the MyATT app and turn data off on any line when they reach your limit.

Thank you!

@genren8293There are some really good apps out there though, depending on your OS/device.  
I started using one of these for my daughter, and between that and a couple of "data talks", she's had zero problems, in quite a few months now.

I have it set to warn her at about 75%, and then again at about 95%, of "her chunk" of our monthly data.  She rarely exceeds the 75% now, by much, apparently the "nudge" was a big help.  

These apps are as close to real-time data as you can get, which is important when doing this type of tracking.

I use the same type of app, when travelling in Canada, to watch all of our usage.

These are per-phone though, mostly, not centrally managed, so it's possible for a user (who's usually the default admin on their device) to override the settings, so they have to have some level of responsibility.

Like @sandblaster noted, this feature was removed from the Smart Limits carrier feature, probably because the non-real-time nature of data made it hard to satisfy customers with its use.  

Android phones have a voluntary cap in settings.  The user can easily change it, but it can be helpful

Any user can check ATT totals by calling *3282# 

data totals by line will be sent by text.  Due to delay in reporting, those totals are not current.

You can manually turn off data.