If I were to place a sizable number of counters on Caller of the Pack and attack with it, do the copies created share its base power and toughness (8/6) or its current power and toughness at the time of the attack?
The copies are 8/6 because it's the printed values that are copied.
706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The “copiable values” are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and “as . . . is turned face up” abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.
If the continuous effect that's boosting the original's power and toughness is from a static ability, and if the copies match the criteria for getting the boost, then (and only then) will they get a boost.
For example, power- and toughness-modifying counters only affect the permanent on which they reside, so they wouldn't affect the copies.
611.2c If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won’t change. (Note that this works differently than a continuous effect from a static ability.) [...]
611.3a A continuous effect generated by a static ability isn’t “locked in”; it applies at any given moment to whatever its text indicates.
611.3b The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.
- For cards, this refers to the values in Gatherer if they differ from the values actually printed on the card. For tokens, it refers to the values given to it when it was created.
Sorry... completely missed the rulings on Gatherer:
Each token copies exactly what was printed on the original creature and nothing else. It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
So it's base power and toughness.