How many times can you use the axe's active ability? I have this question, because the card itself does not limit the number of uses, but if we agree to use it as many times as you have sanity to spend, how should we play cards like fishing net, which does not limit itself, but its repeated use should lead to an absurd advantage?

When in doubt, always take the wording literal: in this case: "you may spend 2 Sanity to reroll any number of dice [...]". It doesn't say, like, "for each 2 Sanity spent, reroll any number [...]".

So, just once per Combat-Encounter. BUT you may use it more than once in one turn, for example mutliple times in your Encounter Phase, in case you have to fight more than one Monster on your space, and another time, in case you're getting ambushed during Mythos Phase.

That would be my interpretation.

IIRC you can only use the abilities of items once per monster combat.

It's under Possessions in the Ref Guide (pag. 9). The last bullet point reads:

If an effect does not list a limit, it may be used only once per instance of the triggered event. For example, if an Asset allows an
investigator to recover 1 additional Health when he performs a Rest action, he may only use that effect once per Rest action.

So, yeah, you can't use that item twice against the same monster in the same round, but you can use it on several monsters during the same encounter phase

Ok, thank you! I would also like your opinion regarding a question I read on Board Game Geek:

I know that if you have an asset that grants +1 to Str in combat, and an asset that grants +2 to Str in combat don't stack, but in our game last night I had one asset that granted +1 Will AND +2 Str, while another asset I had granted +3 Str.

The question is, do I get the +3 Str AND the +1 Will (because they are different skills)?

The point here is that you don't check both skills at the same time, but one at the time. So, Rules Ref (pag 11, under Tests, 4th bullet point):

He may use one effect that provides a bonus (for example “Gain +3 Will"). If multiple effects provide a bonus, he uses only the highest bonus.

Hence, when you check Will, you use the asset giving you the +1. When you check Strength, you may choose another asset.