How many charges there usually are in stafs, magic rings (invisibility) etc? I'm afraid that I'm sparing too much everything cause they never run out...

Don't be afraid! As my ex-boyfriend once said to me, "You never fully appreciate something until you've abused it enough to make it go away." Slug away, tiger!

Really, though- sagely sound-bytes aside, the number of charges on items varies according to where they came from and all that hub bub, bub.

AFAIK there's no way to tell for certain how many charges are left on wands, etc, unlike BG2 where you have a nice number appearing in the quick slot.
One way to tell roughly is to bring the item in question to a merchant and see how much he offers for it on the sell screen. The more it will fetch, the more charges there are.
In BG2, you could "recharge" the wand by selling it and buying it back from a merchant. Not sure if this will work in BG.
I read somewhere not to worry about having to save these wands for that "tough fight" as there are wands a-plenty in the game. Can't vouch for this though.

There are enough wands.

Once you get to Baldur's Gate you can buy them in the Sorceror's Sundries (It is in the first area)

The number of charges is proportional to the cost of the wand. Go to a merchant and see how much he offers you for it. Then use one charge and see how much he offers you now. The difference between the prices is the cost of one charge, so now it's easy to figure out how many charges are in the wand.

OK, thanks. From this moment on I'll 'wand the foes'! (and hopefully avoid some reloads)

Just use the rare wands (such as Monster Summoning) sparingly--Fire, Frost, Lightning and Magic Missile are plentiful, though, so be as trigger-happy as you want with them.