Coelocanth: The max is 18 in every stat. What your minimums are depends on the class (and, I believe, race) you choose. So those minimums will change the max number you'll have to spend in the pool.
The maximum for the base stats is 18 but racial modifiers can push a stat beyond that. A dwarf for example can have 19 constitution and some races have limits on their stats.
Minimums are determined by both race and class. A fighter needs at least 9 strength and a priest needs at least 9 wisdom for example. In both cases you'll want higher stats than the minimum anyway so it doesn't matter a lot.
According to the 2nd edition PHB the following min/max abilities are used. I have adjusted them for racial modifiers.
Dwarf ---- str 8/18 - dex 3/17 - con 12/19 - int 3/18 - wis 3/18 - cha 2/16
Elf -------- str 3/18 - dex 7/19 - con 6/17 --- int 8/18 - wis 3/18 - cha 8/18
Gnome - str 6/18 - con 3/18 - dex 8/18 --- int 7/19 - wis 2/17 - cha 3/18
Half-Elf - str 3/18 - dex 6/18 - con 6/18 --- int 4/18 - wis 3/18 - cha 3/18
Halfling - str 6/17 - dex 8/19 - con 10/18 - int 6/18 - wis 3/17 - cha 3/18
Humans have 3/18 for very stat
Only Fighters, Rangers and Paladins can have exceptional strength and only if their modified strength is 18. As a result, Halflings can never have exceptional strength under the standard rules.
Classes also have minimum requirements and have limits on which races can chose them
Fighter --
str 9 (all races)
Paladin -
str 12 - con 9 - wis 13 -
cha 17 (Humans only)
Ranger -
str 13 -
dex 13 - con 14 -
wis 14 (Human, Elf, Half-Elf)
Mage ----
int 9 (Human, Elf, Half-Elf)
Cleric ---
wis 9 (all races)
Druid ----
wis 12 -
cha 15 (Humans and Half-Elves)
Thief -----
dex 9 (all races)
Bard -----
dex 12 - int 13 -
cha 15 (Humans and Half-Elves)
Specialist Mages need int 9 and have one additional ability requirement. Not all specialisms are available to every race.
Abjurer ---------- 15 wis (Humans only)
Conjurer -------- 15 con (Humans and Half-Elves)
Diviner ---------- 16 wis (Human, Elf, Half-Elf)
Enchanter ----- 16 cha (Human, Elf, Half-Elf)
Illusionist ------ 16 dex (Humans and Gnomes)
Invoker ---------- 16 con (Humans only)
Necromancer - 16 wis (Humans only)
Transmuter ---- 15 dex (Humans and Half-Elves)
Non-human races can chose to multi-class. This means they can select more than one class at character creation. Experience earned is divided evenly across all selected classes. At level-up a multi-classed character will receive less hitpoints. If you picked 2 classes you will get half the hitpoints and if you selected 3 classes you will only get a third. Your constitution bonus if you have it will be divided too. A multiclassed character must meet the minimum requirements of each class. The following multi-class options are available per race.
Dwarf: Fighter/Thief - Fighter/Cleric
Elf: Fighter/Mage - Fighter/Thief - Mage/Thief
Gnome: Fighter/Cleric - Fighter/Illusionist - Fighter/Thief - Cleric/Illusionist - Cleric/Thief - Illusionist/Thief
Halfling: Fighter/Thief
Half-Elf: Fighter/Cleric - Fighter/Thief - Fighter/Mage - Cleric/Ranger - Cleric/Mage - Thief/Mage - Fighter/Mage/Cleric - Fighter/Mage/Thief
For all of the above combinations a Half-Elf may select Druid instead of Cleric.
Humans cannot multi-class but can dual-class instead. Dual classing means that a character stops advancing in his/her current class and start on a new class. All old class abilities with the exception of hitpoints will no longer be available until the level of the new class exceeds the level of the old class. At that point all previous class abilities become available again. A character does not earn hitpoints from the new class until it reaches the level of the old class. dual-class combinations are restricted to any of the combinations possible for non-human races. This means that Paladins and Bards cannot dual-class. Mage specialisms are treated as the regular Mage class for the purpose of dual-classing.
Because dual-classing is very powerful there are additional requirements for the stats. A character needs at least 15 in the prime requisites of the previous class and at least 17 in the prime requisites of the new class. Prime requisites have been underlined in the class table above. Specialist Mages have only int as a prime requisite.