There are thousands if not millions of First and Secundae, there are also half-bloods who live there as well. All members of the first have the same power sets to varying degrees and some have a special skill that deals with their nature. The First had a war with each other that spanned the cosmos, in which they destroyed many worlds, Altwaal got pissed and forcibly transported every last one of them back to their home world and told them to stop and they did. Members of the first are notorious for stealing power from other beings to add to their own personal stores, they can also call power from across the universe given to them by those that worship them. Ingra created a world that was an exact copy of "heaven" with flying gods and all, as a ruse to test the power of a sigil-bearer, and instantly transformed it into a volcanic wasteland devoid of life. She too was also able to forcibly transport a large number of her people sending them all to their homes. Pyrem seems to be connected to the world they live on so much that weather is dictated by his emotional baggage. The first also have two members Wyture and Enson, who are First, but are also part of a greater being (Danik) who has power that rivals Charon and Solusandra. Solusandra apparently lived with Altwaal for who knows how long before she created the First to be his new companions. A Secundae named Animora was powerful enough to wreck havoc on a world populated entirely by sorcerers and they only managed to imprison her. Trenin can create virtually any kind of tech and weapon he can conceive. Gannish is a truthseer and nothing can be hidden from him, Persha can sense through the senses of others without them knowing and control shadows to use as portals she has also matured into power that rivals her mother (Ingra) and is more powerful than many others. Seahn is a beast and a killer who can call power from the earth beneath him, Orium is a nut job who can see the future and control the elemental forces of water to drown others in his insanity (which stems from his mind constantly shifting through past, present and future events). Yala is a warrior goddess and trains just about everybody in house Dexter in combat, because she's that good and she is very prone to bloodlust the joy of battle and all that good stuff. Emphers powers grow as the suffering of others around him does, Evinlea, one of the weaker members has the power to shake worlds and demonstrated this when she wanted to get Charons attention. Braag is an engineer and can create prisons to hold specific gods. Did I mention there are probably millions of them, and I personally place many of them above herald-level
So does Asgard has millions, let us not forget that Odin smite a whole race of Gods that spawned Mangog, and brought them back. Asgard itself is vast, with other Gods, Hela a death god, Sutur w/twilight sword and his warriors, Ymir and his warriors, durok the Demolisher, Ulik and his race, Mangog, Midgard Serpent, then Asgardian Destroyer, Loki, Thor, Odin in Asgard is God. Odin ran shock waves through out the multi-verse if he is going all out, Asgard has to much of advantage especially in there own realm.
Who said the battle takes place in Asgard? it just takes place, the first don't have any weaknesses away from their home except in Negation space and the only example we have of that is Evinlea who is a lesser first, this only effects their power to manipulate ambient energies fully, it takes nothing from their power only their control over it and they are still vastly superhuman and virtually immortal with all their psionic powers intact. Ingra has showed many times that what weakens certain first has no effect on the more powerful ones. The first can also pool their powers together to empower one or others as the channel for that power. The first were created because the universal energies needed to power god-like beings was ebbing, they were made to fight epic battles to create more power through conflict, the more they fight the more energy they create to use.
So will Altwaal. Enson and Wyture being aspects of one of the creator gods (and wield greater power than all the first) also have the power to allow the others to use the powers of Asgard as easily as any asgardian since all aspects of danik have his power and he was able to give others full access to the chaotic Negation energies via Gammids sigil. The energies Odin use are not outside the scope of marvel they are just cemented in Asgard so that he can fully draw on them, he'll be to busy with Altwaal and the weapons to stop many first from slaughtering other asgardians.