LCU is a tool that enumerates the available DLUs in the Unified CM cluster and without changing the state of the system, suggests the corresponding UCLs required.
version 10.5 and the latest version are using the same licensing. you will get all the existing license details from the PLM. in version 12, its smart licensing. you need a smart account to deposit all the licenses.
if you are asking about the upgrade path, go through Upgrade and Migration Guide.
Mandatory reading regardless of what upgrade path you choose, all for target version
A) Upgrade and migration guide
B) Release Notes
C) Compatibility matrix (depending on product might be a standalone doc, or within any of the other docs I mention)
D) README from ova
E) HW/ESXi compatibility
After reading all of the above, you'll have all the information to plan the upgrade.
12.5 has a pre-upgrade and post-upgrade COP files that check on a few things that could make the upgrade fail, and that is also explained in the above documentation.