Hey JDM885 welcom to MFK!
To answer this question, we are going to need your tank size, as well as the what kind you have and also what one you plan on getting.
its a 60 gallon oceanic tank.. its a senegalus bichir, the plain grey one.. i want to get the same type, just dont know if the older which is about 5 inch. will show agression to the younger one.
no problem unless the new ones bite size.
i currently have 13 in a 4 by 3 by 3 with other fish.
for a 60,depending on filtration,maybe 10 of the smaller species.