How is the diploid chromosome number maintained at mitosis?
The terminology of "chromosome" during this process is confusing, but here it is:


There are 46 chromosomes in the cell (23 pairs)
Before division, each chromosome has two sister chromatids, joined in the middle (92 chromatids in total)
During division, the chromosomes split and each daughter cell gets 46 chromatids, one from each chromosome
These chromatids are now known as chromosomes
Each one of these new chromosomes only has one "chromatid"
During interphase, the new chromosomes replicate and now they have two chromatids each, joined in the middle


So whether a chromosome has one or two chromatids it is still called a chromosome. I hope this helps.

The following link contains a more complete explanation of mitosis. The first picture explains this point quite succintly: http://biology.tutorvista.com/cell/mitosis.html