Portal is a small set with 85 commons, 55 uncommons, 55 rares, and four variations of each standard basic land. There are extra commons compared to a normal small set of this era. The set was printed in English in the USA. The existence of English Belgian-printed Portal is unknown. 
Portal was sold in 15 card booster packs that contain an additional tip card. Booster boxes have 36 packs.
The US printing uses sequential collation and 10 × 11 sheets. Packs are back-facing. The first eight cards in the pack will include the tip card, two basic lands, four commons, and one uncommon. The tip card can be at any position, but relative to the tip card, the other cards are always in the same order with the lands followed by commons followed by uncommon. This is cyclic, so, for example, if the eighth card is the tip card, the first two cards will be lands. These cards can come from one of five runs, each based around one of the five allied color combinations, so I refer to them as seeded cards.
After this 8 card run, there is a rare, then 2 uncommons, and 5 commons.
|8 Seeded Cards||1 Rare||2 Uncommons||5 Commons|
There are pictures of all the sheets. 
The commons and uncommons that appear in the seeded runs are completely separate from the other commons and uncommons. Each seeded run has three commons and two uncommons of each color. These may appear different numbers of times. Also, the seeded runs are different lengths, and don't all appear equally. There are three sheets for the seeded runs. Two have two runs each — one in the top six rows and one in the bottom five rows. The longer runs here must be more likely to appear as they are printed at a higher rate. The last run is printed on its own sheet, so the relative rate is not known. (A reasonable guess it that this sheet is printed at half the rate of the sheets with two runs.)
There are 55 remaining commons. Each is printed twice in a single long common run.
There are 35 remaining uncommons. Most cards are printed three times, but a few cards are printed four times. One card is printed only twice (which is perhaps a mistake). Probably the sheet is just one long run, but I haven't been able to verify this.
The rarities of seeded cards are different than the rarities of other commons and uncommons. Assume that the run that appears on its own sheet (the red and black run) is indeed printed at half the rate of the other seeded sheets. Then we can call a card that appears once in this run S1 (making a card that appears once in one of the other seeded runs S2). Then S1 is 11/13 as likely as U1. This means S2, S3, and S4 uncommons are slightly rarer than U2, U3, and U4 respectively. For commons, S1 is 11/65 as likely as a common. This means S8, S9, and S10 commons are significantly more common than other commons.
The 55 rares are each printed twice on a sheet.
Note that some Portal cards have an alternate version. Many cards contained in the preconstructed starter decks have additional reminder text.
The white and green run, "The Horde," has length 56. It is printed on the top half of the same sheet that has the white and blue run. (Note that the tip card images are currently placeholders.)
The white and blue run, "Air Superiority," has length 48. It is printed on the bottom half of the same sheet that has the white and green run.
The red and green run, "Gargantuans," has length 56. It is printed on the top half of the same sheet that has the blue and black run. (Note that the tip card variants are hard to make out in the sheet picture, so I have reconstructed what I could and assumed the remaining tip cards alternate as is the case in most of the other runs.)
The blue and black run, "Card Domination," has length 48. It is printed on the bottom half of the same sheet that has the red and green run.
The black and red run, "Fiery Doom," has length 104. It is printed on its own sheet.
The common sheet contains 55 cards each twice. The cards follows a repeating color pattern of white, red, blue, green, black.
The uncommon run contains 29 cards each three times, six cards each four times, and one card twice. The cards follows a repeating color pattern of white, black, red, blue, green.
The rare sheet contains 55 cards each two times.
 The "Compilation of Manufacture Information for MTG Products" thread on the Magic Librarities Forum lists English Belgian-printed Portal as uncertain. I also did not find any during my research.
 The "Compilation of Uncut Sheets" thread on the Magic Librarities Forum has pictures of all the sheets.