I used Ubuntu 16.04 LTS 32 bits. I have been notified that i can upgrade to 18.04. My question is that one: Which version of Ubuntu 18.04 will be installed, 32 bits or 64 bits?
First of all, if the CPU is 32-bit, you can't install a 64-bit OS on it.
Judging by the answers on Is it possible to "upgrade" from a 32bit to a 64bit installation? and How can I switch a 32-bit installation to a 64-bit one?, it is very difficult to switch from 32- to 64-bit, so I can't imagine the upgrade would do it for you.
According to answer on this question you can upgrade 16.04 LTS to 18.04 LTS with 32-bit support:
There is a bit of confusion in this area.