Please format your code using the suggestions here. Also the title of your post should be a question that you would ask a friend.
One option is to not use the numbering provided by the page() function and instead add the page number to the background parameter of page():
#set page(
paper: "a4",
margin:(top:2.5cm,left:3cm,right:6cm,bottom:2.5cm),
background: place(
bottom + center,
dy: -1em,
context counter(page).display()
),
//Remove these next two if numbering in the background
number-align:bottom+center,
numbering: "1",
)
//Visualize the margins
#box(
width: 100%,
height: 100%,
stroke: 1pt
)