I don't know the apartment number/code of my building.
I don't know the apartment number/code of my building.
The 「Apartment number (code)」 of your building is written on the details of the building in your rental contract documents.
※The description may vary depending on the contract type, but in Japanese it is usually described as 「AP番号」「APNO」「アパートNO」.

If you signed a digital contract instead and don't have the contract papers, you can access the Tenant's exclusive Website 「&Leo」 and select the option (check/change the information registered)【ご入居者様情報確認/変更】 (only available for rental contracts under the tenant's name).
It is also shown on the details of the building that is shown on the information board.