Asslamualikum ustadh I have an irregular menstruation cycle i have already got my period in the month but after 20 days in the same month I again saw reddish brown spotting, whether or not is it considered my period, if yes how many days should i wait for doing ghusal if I dont see the blood

If what you saw is real period blood, it is a period even twenty days later, since there is no minimum length of purity between two periods, and you make ghusl as soon as it stops and the cotton comes out clean. If it is only brownish spotting after you had already become pure, it is not a period, so you wash it off, make wudu and pray.