Transfer number?

Codes are one time use only. But you can reissue a new one as many times as you need. I myself have done it transfered from nox on my laptop to my phone. Just make sure you remember/write down or save the given code and you password for it. The issued code does not affect the account in any way except when you change devices. Also just some advice if you roll for you favorites reissue your code, as far as I know DW retrieves accounts but only until the latest tranfer codes. Hope this helps

It will still stay there and transfer code is permanent until you've used it.
I have been transfering acc pretty often and as long you remember the password and transfer code no problem will occur.

Transfer Code is a must for every FGO player and no exception, keep that in mind.

For transfer codes, it is as others before me said.
One code (and the password that you used to create it) can last for an indefinite amount of time, but are also one-time use codes. After you've used one, you can just create another one. And it is preferable to immediately secure your account by creating a code after the transfer because you never know what kind of stuff can happen to your new device or game. It's better to be safe than sorry.

Make sure that you save your pass + code in notepad, cloud, screenshot it, write it on a note or something, put it in your wallet, etc. Stuff can happen and keeping it in just one format on the same device that's about to die can spell disaster for you even if you backed it up.

Secondly, make sure you remember the date when you created your account, when was the last time you played, your support team comp (+CEs), your current party (+ CEs), and some of your servant levels (pref. in your active party), your account ID. These questions may be helpful for the support if there are problems with your code.

The second part obviously is just for an extra layer of security, it's not something you need to keep in the back of your head if you're constantly playing. Your party/support list is obviously going to change from time to time.