Do you get a counter for each Tempered Sliver on the battlefield
The sliver in case would, in fact, get 2 +1/+1 counters.



This is because of rule 113.2c, which I quote, with the relevant bit emphasized:


  113.2c An object may have multiple abilities. If the object is represented by a card, then aside from certain defined abilities that may be strung together on a single line (see rule 702, “Keyword Abilities”), each paragraph break in a card’s text marks a separate ability. If the object is not represented by a card, the effect that created it may have given it multiple abilities. An object may also be granted additional abilities by a spell or ability. If an object has multiple instances of the same ability, each instance functions independently. This may or may not produce more effects than a single instance; refer to the specific ability for more information.


The part in bold font means that your slivers (including your Tempered Slivers) have, besides their other abilities, two instances of 


  "Whenever this creature deals combat damage to a player, put a +1/+1 counter on it". 


Both instances will trigger independently, giving your slivers two +1/+1 counters.