It really depends on the configuration of your web server. There is no ‘correct’ answer.
The uploads directory needs to be writable by the user that your web server runs under (even this assumes that you are running a LAMP web server, IIS will be different).
For 755, the first number 7, means that the owner of the directory can read, write and browse the contents of the directory. The second number is the group of the directory, and the third number is everyone else.
You should be able to ask your web host what user the web server is running as and then change the ownership of the uploads directory to that user. Then as long as your permissions on the directory are set to 7** you’ll be good.
Your web host should help you if you tell them you need the uploads directory to be writable by the user the web server is running as. If they can’t, change hosts, because any good host will do this for you.
Also as a site note, some web server configurations don’t allow 777, and allow a maximum of 755 as a security issue (ie group and other users can’t ever write, they can only read).
https://codex.www.ads-software.com/Changing_File_Permissions
