Hi, why must let it be 6u at first, can it 4u or 5u at first? Please advise, thank u

I decided on 6u because I want a common multiple of 2 and 3.
Why?
Sell 1/2 of the chairs. That means I want chairs to be divisible by 2.
What’s left is 1/3 of the total. That means my total should be divisible by 3.

So I chose 12u to be my total (divisible by 3) and 6u to be my chairs and tables (divisible by 2).

Now I understand, thank u so much