How many times can I use my transfer code?

It clearly says one time only, but as someone who lost his account due to this kind of experiment don't try it much you'll get an error or something even if you created a new transfer code and put back on your phone from another device. The way to solve this however is to accept to create a new account (in your main phone), and then put the transfer code.
Sorry for being confusing, but yeah, you could only use it once and you need to create a new transfer code for every transfer.

Yes you can if your phone is xiaomi with dual apps feature

You can only use each transfer code once, but you can create a new transfer code as many times as you want, AFAIK.

I don't know exactly what you're trying to do, but if you're just looking to play around on a second account for a bit, why not use an emulator?