That would depend on how much alike "very much alike" is :-). (E.g. would they map unto 200 different DB tables, or just need a type ID in a column?)
I have not done 200 Entity EJBs in JBoss (or anywhere else) but in general i think that it can take fairly much of performance if you do that. If it is possible, and you do not loose too much of other things (ease of understanding and such) I would lean towards having only one Entity Bean.