Because the Vessel Is Vast
The simile turns around. Seen from the ocean's side, the question is not how much rain falls but why nothing is overwhelmed — and the answer is four characters long. Then Vimuktacandra asks for a number, and is given a comparison instead.
Why Nothing Overflows
One dragon king or measureless dragon kings, all raining at once in a single instant — and the ocean's capacity is not strained, for a reason the sūtra states without any mystery about it.
佛子！譬如大海，能安、能受、能攝、能持一大龍王所霔大雨；若二、若三乃至無量諸龍王雨，於一念間一時霔下，皆能安、能受、能攝、能持。何以故？以是無量廣大器故。住法雲地菩薩亦復如是，能安、能受、能攝、能持一佛法明、法照、法雨；若二、若三乃至無量，於一念頃一時演說，悉亦如是。是故此地名為：法雲。」
Child of the Buddha! It is like the great ocean, which is able to settle, to receive, to gather in and to hold the great rain poured down by one great dragon king; and if two, or three, and on up to measureless dragon kings' rains were to pour down together in a single instant, it would be able to settle them all, to receive them, to gather them in and to hold them. Why is this so? Because it is a measureless and vast vessel. A bodhisattva abiding on the Ground of the Dharma-Cloud is likewise able to settle, receive, gather in and hold one buddha's light of Dharma, illumination of Dharma and rain of Dharma; and if two, or three, and on up to measureless buddhas were to expound together in the space of a single thought, it would be just the same. It is for this reason that this ground is named the Dharma-Cloud."
以是無量廣大器故 — because it is a measureless and vast vessel. The explanation is about the receiver, not the rain. Nine grounds of practice are being described here as the enlarging of a container, and the tenth ground's achievement is that nothing arriving can now be too much for it.
A Number Refused
The question is asked plainly and the answer begins by declining to count. What follows is a thought-experiment whose whole purpose is to build a very large number and then set it aside.
解脫月菩薩言：「佛子！此地菩薩於一念間，能於幾如來所安受攝持大法明、大法照、大法雨？」金剛藏菩薩言：「佛子！不可以算數能知，我當為汝說其譬諭。佛子！譬如十方各有十不可說百千億那由他佛剎微塵數世界，其世界中一一眾生皆得聞持陀羅尼，為佛侍者，聲聞眾中多聞第一，如金剛蓮華上佛所大勝比丘；然一眾生所受之法，餘不重受。佛子！於汝意云何？此諸眾生所受之法為有量耶？為無量耶？」解脫月菩薩言：「其數甚多，無量無邊。」
Vimuktacandra Bodhisattva said: "Child of the Buddha! A bodhisattva on this ground, in a single thought, is able to settle, receive, gather in and hold the great light of Dharma, great illumination of Dharma and great rain of Dharma in the presence of how many Tathāgatas?" Vajragarbha Bodhisattva said: "Child of the Buddha! It cannot be known by any reckoning; I shall set out a comparison for you. Child of the Buddha! Suppose that in each of the ten directions there were worlds as numerous as the dust-motes of ten ineffably many hundreds of thousands of koṭis of nayutas of buddha-lands; and that every single being in those worlds had attained the dhāraṇī of hearing and retaining, and was an attendant of a buddha, foremost in learning among the śrāvaka assembly, like the bhikṣu Great Victor in the presence of the Buddha Atop the Vajra Lotus; and that the dharma received by any one being was not received a second time by another. Child of the Buddha! What do you think? Would the dharma received by all these beings be limited, or would it be measureless?" Vimuktacandra Bodhisattva said: "Their number would be exceedingly great, measureless and boundless."
金剛藏菩薩言：「佛子！我為汝說，令汝得解。佛子！此法雲地菩薩，於一佛所一念之頃，所安、所受、所攝、所持大法明、大法照、大法雨、三世法藏，前爾所世界一切眾生所聞持法，於此百分不及一，乃至譬諭亦不能及。如一佛所，如是十方如前所說，爾所世界微塵數佛復過此數，無量無邊，於彼一一諸如來所所有法明、法照、法雨、三世法藏，皆能安、能受、能攝、能持，是故此地名為：法雲。
Vajragarbha Bodhisattva said: "Child of the Buddha! I shall explain it for you, so that you may understand. Child of the Buddha! As for the great light of Dharma, great illumination of Dharma, great rain of Dharma and treasury of the Dharma of the three times that a bodhisattva of this Ground of the Dharma-Cloud settles, receives, gathers in and holds in the presence of a single buddha in the space of a single thought — all the dharma heard and retained by all the beings of all those worlds just described does not amount to one part in a hundred of it, nor to anything a comparison could reach. And as with one buddha, so throughout the ten directions as described before: buddhas as numerous as the dust-motes of all those worlds, and beyond that number, measureless and boundless — in the presence of each and every one of those Tathāgatas, all their light of Dharma, illumination of Dharma, rain of Dharma and treasury of the Dharma of the three times, they are able to settle, to receive, to gather in and to hold. It is for this reason that this ground is named the Dharma-Cloud.
然一眾生所受之法，餘不重受 — the dharma received by any one being was not received a second time by another. The stipulation is what makes the thought-experiment work: no duplication, so the total is the sum of everything distinct that could be learned by beings beyond counting, each of them an Ānanda. Vimuktacandra answers honestly that it is measureless, and is then told it does not amount to a hundredth part.
如金剛蓮華上佛所大勝比丘 — like the bhikṣu Great Victor in the presence of the Buddha Atop the Vajra Lotus. The chapter pauses to name a particular monk in a particular buddha's assembly as the standard of learnedness. It is the only named individual in this whole ground, and he appears only to be exceeded.
Part 479 Complete
This continues Dharmameghā, the Ground of the Dharma-Cloud, the tenth and last of the Ten Grounds, in fascicle thirty-nine. Part 480 gives the ground its name twice more — the bodhisattva who raises a cloud of great compassion, sounds the thunder of the great Dharma, makes the penetrations and fearlessnesses his lightning, and rains nectar on measureless lands; and the one who, in world after world, shows a buddha's whole career from the Tuṣita descent to nirvāṇa.