How can I arrange bricks?

You can use two for loops to move over the space in a grid.

Here's what we'll use to start:

for x = 0, 3 do
    for y = 0, 3 do
        print("(",x,",",y,")");
    end
end

Output:

( 0 , 0 )
( 0 , 1 )
( 0 , 2 )
( 0 , 3 )
( 1 , 0 )
( 1 , 1 )
( 1 , 2 )
( 1 , 3 )
( 2 , 0 )
( 2 , 1 )
( 2 , 2 )
( 2 , 3 )
( 3 , 0 )
( 3 , 1 )
( 3 , 2 )
( 3 , 3 )

So this covered a grid of spaces.

Since the bricks are 10 wide in each way, their centers are 10 apart. That means we just need to multiply the grid we made by 10.

local parts = model:GetChildren() -- the parts we are placing
local i = 0
for x = 1, 4 do
    for y = 1, 5 do
        i = i + 1;
        -- ith part
        parts[i].CFrame = CFrame.new( x * 10 , 0 , y * 10 )
    end
end