There are innumerable forms. But Arjuna knew that Krsna is the original Personality of Godhead assuming His temporary universal form. He is now asking to see the form of Narayana, a spiritual form. BG 1972 purports

Expressions researched:
"There are innumerable forms. But Arjuna knew that Kṛṣṇa is the original Personality of Godhead assuming His temporary universal form. He is now asking to see the form of Nārāyaṇa, a spiritual form"

Bhagavad-gita As it is

BG Chapters 7 - 12

In the Brahmā-saṁhitā it is stated that the Lord is eternally situated in hundreds and thousands of forms, and the main forms are those like Rāma, Nṛsiṁha, Nārāyaṇa, etc. There are innumerable forms. But Arjuna knew that Kṛṣṇa is the original Personality of Godhead assuming His temporary universal form. He is now asking to see the form of Nārāyaṇa, a spiritual form.

BG 11.46 (1972), Translation and Purport

O universal Lord, I wish to see You in Your four-armed form, with helmeted head and with club, wheel, conch and lotus flower in Your hands. I long to see You in that form.

In the Brahmā-saṁhitā it is stated that the Lord is eternally situated in hundreds and thousands of forms, and the main forms are those like Rāma, Nṛsiṁha, Nārāyaṇa, etc. There are innumerable forms. But Arjuna knew that Kṛṣṇa is the original Personality of Godhead assuming His temporary universal form. He is now asking to see the form of Nārāyaṇa, a spiritual form. This verse establishes without any doubt the statement of the Śrīmad-Bhāgavatam that Kṛṣṇa is the original Personality of Godhead and all other features originate from Him. He is not different from His plenary expansions, and He is God in any of His innumerable forms. In all of these forms He is fresh like a young man. That is the constant feature of the Supreme Personality of Godhead. One who knows Kṛṣṇa at once becomes free from all contamination of the material world.