how many Seachem Purigen i need for a 90G tank?

There's a 100mL bag you can buy with the Purigen in it. A couple of those ought to do it.

I think the recommended dosage is 1mL per gallon, but a little extra won't hurt anything.

I'm a Purigen fanatic.

i bought a liter,so if i put all of them in the filter .that will be too much right?

That does sound like a lot.

I don't know what you're planning on putting the Purigen in, but the only thing that has worked for me, without releasing all of the little white balls all over the tank, is the Seachem "Bag." It's specificly made for the Purigen and holds about 250mL.

A whole liter in a 90 gal. is a lot. I don't know that it'd have an adverse effect, but you could cut that down to 250mL and save the rest for later. You can recharge the Purigen, but it's more of a pain than it's worth to me. I just replace it.