i think one argument can have only 1 assumption.
the assumption will connect premise to the conclusion.
The assumption will be necessary to validate the conclusion.

duongthang wrote:in one argument we can have many assumptions. Is that right? and because we have many assumptions, we can have many strengthener and weakeners. Is that right?

PLEASE, GIVE ME ONE ARGUMENT AND ITS 2 ASSUMPTIONS. I do not understand this.

Hi,
Yes. An argument can have any number of assumptions.
An advertiser: "Our new advertisement in the newspaper will increase the sales of our product"
The assumptions are "People see advertisements in the newspaper", "People buy products based on advertisements", so on. All such hidden statements are assumptions.

Strengthen questions ask you to support the argument in any way possible. This type of answer has great range, as the additional support provided by the answer choice could be relatively minor or major. Speaking in numerical terms, any answer choice that strengthens the argument, whether by 1% or by 100%, is correct. That is the reason, question clearly states "which of the following if true, most/least strengthens the argument"

Definitely an argument can have well as many assumptions as you deem fit.
while considering a weaken question, Your question thus always states choose the option that would most weaken the argument...

here even though the author may have assumed a couple of things ,which we all do when we make conclusions.... simply not possible to state every possible case.... we must check the option which the entire argument depends on that is if you negate that particular assumption the conclusin should sound RIDICULOUSLY STUPID!