"Sacrifice a Creature" cost on as an ability: is there a limitation to how many times you can use it each turn?
You can activate an ability as many times as you like as long as you can pay the cost.  The  only restrictions on activating an ability are listed in rule 602.5:

602.5. A player can’t begin to activate an ability that’s prohibited from being activated.
602.5a A creature’s activated ability with the tap symbol ({T}) or the untap symbol ({Q}) in its activation cost can’t be activated unless
the creature has been under its controller’s control since the start
of their most recent turn. Ignore this rule for creatures with haste
(see rule 702.10).
602.5b If an activated ability has a restriction on its use (for example, “Activate this ability only once each turn”), the restriction
continues to apply to that object even if its controller changes.
602.5c If an object acquires an activated ability with a restriction on its use from another object, that restriction applies only to that
ability as acquired from that object. It doesn’t apply to other,
identically worded abilities.
602.5d Activated abilities that read “Activate this ability only any time you could cast a sorcery” mean the player must follow the timing
rules for casting a sorcery spell, though the ability isn’t actually a
sorcery. The player doesn’t actually need to have a sorcery card that
they could cast.
602.5e Activated abilities that read “Activate this ability only any time you could cast an instant” mean the player must follow the timing
rules for casting an instant spell, though the ability isn’t actually
an instant. The player doesn’t actually need to have an instant card
that they could cast.

So in the case you've listed above where you can recur and sacrifice a creature endlessly, go wild (note that you can't choose to activate something an infinite amount of times, you do have to pick a number, but that's a whole different discussion).