Given the likely size of the table I wouldn't say more than three or four Mumakil. Each deserves a certain amount of personal space to dominate the table without making it look too busy or making them look too crowded. Of course you'd want lots of warriors around them. Just not too many giant big things.
I have found my mounted Aragorn (unpainted 3 part metal model) for the Black Gate [pages 80-83] and as many of the figures from Pelennor Fields are also used in this battle - it makes sense to make it the next one in the series. Would encourage me to do some more gluing and painting to, as well fill in a few more gaps in the collection.
Who can muster 5 eagles?
I dare say that Grumpy Phil would like to get his toys out for Siege of Lotherien [pages 76-77], the Mordor forces, we already have and I can roughly double Phil's elves.
there is one for the Dwarves [pages 78-79] but I guess Raff will have to press-gang some 28mm stumpies to fill in the gaps.