Re: How many Valheru? *SPOILER*
I don't believe that there were very many Valheru at any one time, as they spent much of there lives killing each other (with the victor absorbing the fallens power).
So while there might have only been a few score (maybe a couple of hundred at most) individuals, between them (as the Enemy) they contained the power of the entire race (missing only the power of Ashen-Shugar (the most powerful of the race, and the only one not to take part in the creation of the Lifestone and the Chaos Wars) and of the Valheru that had fallen to the (more numerous) Dasati when they accidently invaded the second circle).
Valheru could defeat DreadLords in one on one confrontations.
When the blood lust was on them, the Dragon Host raged throughout the universe, and fought against and destroyed ever other primordial race they encounter.
Only the Aal survived against the Valheru.
The Dragon Host even slew gods. Some gods even eliminated magic from their world in order to save it (well, they converted magic to a solid crystalline form), in an effort to frighten away the Valheru, rather than face the Dragon Host directly.
While the Chaos Wars, which shook the universe, were mainly about the gods fighting between themselves to determine their place in the scheme of things, a significant part seemed to be the battle of the Valheru against these 'new beings'. And when a Valheru was slain in the Chaos wars, its power/spirit rejoined the conflict (rather than just dying and passing into some 'afterlife'. Hence no Dragon Lord resides in the halls of Lims Kragma).
While the Valheru were unable to slay the younger gods of Midkemia, they were enough of a threat that the Gods banished them from the Universe (of Midkemia).
The Dragon Host, in their rage of being exiled, then went on to destroy entire universes, causing uncounted races to flee to new worlds (Kelewan and Midkemia amoungst them) to escape.
Humans that came to Kelewan, across the golden bridge, named the force that had destroyed their worlds (i.e. the Dragon Host) the Enemy, for they had no better name for the Valheru.
The Valheru are a magical race, almost immortal, that even after being defeated by the Gods of their world, and even in death, still had to be exiled to another universe, which they then destroyed, is more powerful than any vague idea about the Enemy.
There was so little detail about the Enemy, that some Black Robes of the Assembly even thought that the Enemy was no big deal, something that the Assembly would be able to defeat. Assuming that they actually believed in the 'myth'.
I would say you were underestimating the power of the Valheru.
And also the power of Macros, who was able to survive against the Enemy.
But then again the Gods of Midkemia were lending a (hidden) hand.