number of substacks- practical limits

[Kurt Kaufman]

Assuming that an individual substack consists of a single card 
containing perhaps 7 or 8 fields, each with <5 KB of text, plus a few 
pop-up menus and buttons, how many substacks can be associated with a 
single mainstack?  Only one substack would be open at any given point, 
but I might have as many as 2000 substacks.
Right now, I have a single data stack with about 800 cards.  Trouble 
is, only one person can access the data at a time.  Would it be 
possible, if the data was divided by substack rather than by card, for 
more than one person to access the different substacks concurrently?

I guess another way of expressing this would be to say that the 
equivalent of a db "record" would be a substack rather than a card.

TIA,
Kurt