I have not seen any fixed limits. It may be a case of try it and see. It really depends what your forum viewers are doing when they are in the forum. If they are reading items then it will be a lot less cpu intensive than if they were all searching or writing messages.
Try asking support and see if they have any ideas, but I should imagine it will be the usual case of there being no restrictions placed by DreamHost until you impinge on the other server users.
You might get no users at all! I had none for my 'Meet Cyril Nibblethwaite' forum, but you may be luckier.
Opinions are my own views and not the views of DreamHost.
Any advice offered by me should be acted upon at your own risk.