Karlie here from the Shopify Support Team! You can actually make the apartment/unit line optional or hidden if you wanted to. I usually keep my own apartment/unit line as optional in my own Shopify checkout, so that customers with an apartment have the choice of entering that information in for shipping purposes. This line is called the 'Address line 2' in your checkout. To either hide this option, offer it as required or keep it optional, you'll want to follow the below instructions:
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!