Can I buy multiple lenses and keep them if I do not open them, will they expire fast ?

Yes, you can buy multiple lenses, and as many as you want because you can keep them for a long time before you open the bottle, but if you have many contact lenses you should check the expiration date on the vial before you use them.

Actually the circle lenses or big eyes lenses can be stored in their unopened packaging bottle up to 5 years since the date when they came out from factory, but whenever you open the bottle, you should use your circle lenses within 1 year.

And 1 year is what contact lens brands recommend, but ophthalmologists advise to wear lenses a maximum of 6 months even when they are sold as 1 year disposable lenses.