This is a topic about the set's anatomy as well as its implications for the Duels card pool. As soon as we get the full spoiler, I could easily see turning it into a topic to speculate which cards we're gonna get in particular.
So, judging from the collector's numbers, we know the following about the set (in collector's numbers):
White: #001 (Angel of Sanctions) - #039 (Winged Shepherd)
Blue: #040 (Ancient Crab) - #075 (Vizier of Tumbling Sands)
Black: #078 (Archfiend of Ifnir) - #114 (Unburden)
Red: #117 (Ahn-Crop Crasher) - #152 (Trial of Zeal)
Green: #158 (Cartouche of Strength) - #193 (Watchful Naga)
Multicolour: #196 (Bounty of the Luxa) - #207 (Temmet, Vizier of Naktamun)
Split cards: #210 (Dusk//Dawn) - #224 (Heaven // Earth)
Artifacts: #225 (Bontu's Monument) - #238 (Watchers of the Dead)
Lands: #239 (Canyon Slough) - #248 (Sheltered Thicket)
Basic lands: #250-#269
So, we know we'll get 39 White cards, but 38 Blue cards at most. Given we'll get 193-195 monocoloured cards, I'll guess we get 38-39 cards in each colour.
That would mean 193 mono-coloured cards, 16 multi-coloured ones, 15 Split cards, 14 artifacts and 11 lands.
The missing card I presume to be the third Mythic, which will be a two-colour planeswalker. If that assumption is correct, the planeswalker is definitely blue, definitely not white and highly unlikely black. It's going to be either a Simic or an Izzet walker. Given the numbers we know I'd lean towards Izzet, but the rare enchantment Bounty of the Luxa rather points into the Simic direction, making it a bit unclear.
We'll have 16 multicoloured cards. I presume we will get an allied Uncommon cycle that hasn't been spoiled yet (to complement the enemy Uncommon Split cycle), a full cycle of double-coloured rares, and the mythic Walker.
We already know we'll get 15 split cards: one monocoloured Rare cycle, one ally-coloured Rare cycle and an enemy-coloured Uncommon cycle.
We know we'll get 14 artifacts. 5 of these are the Uncommon monuments; we also know the uncommon Watchers of the Dead as well as three Rares. I presume one artifact to be Mythic (see below); the other four are common/uncommon.
We also know we'll get 11 lands. 5 of these are the rare Cycling lands; 5 of them will likely be an Uncommon/Common cycle for manafixing (I guess the Deserts; likely enemy-coloured Guildgate-alikes), the final land we can only speculate about (probably
Evolving Wilds seeing how it would have rotated out of Standard with BfZ)
Since we have the exact same number of card we had in Kaladesh (249), I'll guess the Rarities off that set.
Kaladesh had 15 Mythics, 53 Rares, 80 Uncommons, 101 Commons. If we assume the same numbers for this set, we'll get the following:
Mythics: We know there'll be 5 monocoloured Gods, and we know one non-God non-Walker Mythic from each colour. We know Gideon and Lili, we know there'll be a third Walker (which I made my assumptions about), and I think we'll see another Mythic in either Red or Green (whichever colour the Walker is not in).
This leaves us with one final Mythic; and I guess it's going to be an artifact (probably one that uses brick counters)
Out of the 53 Rares, we know the two split cycles (5 cards each), and I'll presume the 10-card multicolour cycle. We also know 3 artifacts.
This'd leave us with 6 Rares per colour (not counting the mono-coloured split cards), which seems like a decent number considering the set's size. Note that we know 4 Rares of some colours already.
Out of the 80 Uncommons, we know the split cycle (5 cards) and the Monument cycle (5 cards). I also presume an allied multicolour cycle (5 cards) and we don't know whether the land cycle will be common or uncommon (I'll assume Common, see below). Given that we know Watchers of the Dead, I will assume 4 artifacts or 3 artifacts plus the non-cycle land to be Uncommons (5 cards).
This leaves us with 12 Uncommons per colour (we know 7 from some colours).
None of the 101 Commons will be split- or multicolour-cards; and only one artifact or non-cycle land will be common. This is why I presume the land cycle to be Common rather than Uncommon, as this still leaves us with 19 Commons per colour (we know 11 in White).
As for Duels, we'll see the usual big set fare: 10 Mythics, 35 Rares, 52 Uncommons, 61 Commons.
For the Mythics, I'll assume the Gods, the Walkers and 2 assorted Mythics (likely the artifact as well as one of the mono-coloured Mythics; I could see the artifact being left out which would lead us to 2 monocoloured Mythics)
For the Rares, I think none of the artifacts are going to make it (maybe 1 if we don't get the full Rare cycle). The lands will be in only if some of the split cards are left out I believe; as we won't see them cut all of the multi-coloured Rares (I hope we don't get the full cycle, though).
I believe we'll get 4 mono-coloured Rares per colour (maybe including the split cards, maybe not) and 15 of the multicolour/split cards.
We don't know enough Uncommons/Commons to make final assumptions which ones are in and which are not.