There are no limits in the rules
Traits can, and should, be used whenever a player (or the Loremaster) think that it should apply to that situation. Considering how small their benefits are, they are hardly unbalancing, but they are vital to each character's progression.
As described under Trait Etiquette (pg.92):
Players can invoke a Trait when they think it
applies to the situation at hand (sometimes, the
Loremaster himself may invoke one of a hero’s
Traits). To do so, they briefly explain why they
think the Trait should give their character an
advantage. If nobody at the table objects, then
the player resolves his chosen course of action. If
anyone finds the argument for invoking the Trait
questionable, the Loremaster adjudicates.
The advantages conferred by Traits aren’t
powerful enough to unbalance the game, so players
and Loremasters are advised to avoid discussing
the validity of a Trait at length. A well-detailed
or entertaining explanation adds to the enjoyment
of the game, and should earn the player the benefit
of the doubt. In all cases, the Loremaster’s word is
final.
As much as the Loremaster's word is final on whether or not a Trait should apply on a situation, there are no limitations in the rules about how many times a Trait may be used, as long as nobody at the table is contrary that it should apply to the situation at hand. On the contrary, the Loremaster is encouraged to ask for a player to use one of his Traits so they can receive an adequate amount of Advancement Points (pg.212):
Loremasters should consider that the Advancement point costs for raising Common skills (see page 195) have been calculated considering that a companion should gain an average of 4 Advancement points for each session of play (more in the first session, as checks are easier to get when all boxes are empty).
If the players are constanlty gaining less points, then the Loremaster should encourage them to make a more profitable use of their Traits – for example by keeping a list of which ones are represented in the group and pointing them out during gameplay.
The Loremaster may be thinking about Flaws
Flaws does have a limitation of only use per scene (not session), as their consequences are worse for the players, as described under Invoking Flaws (pg.226)
Using rules similar to those governing the use of Traits,
the Loremaster may invoke a character’s Flaw whenever
it would have a negative effect on the situation at hand.
In other words, it must be reasonably plausible that the
character would have fared better if he didn’t possess the
negative feature depicted by the Flaw.
T
he Loremaster may invoke a flaw to apply a Feat Re-roll
or a Failure Aggravation. Usually, the Loremaster cannot
apply the effects of the same Flaw more than once during
the same scene, and certainly not to the same action.
