I am copying multiple lines from different parts.
I can for example do
3yy
to copy the 3 next lines.
Then I would like to scroll a bit and add 5 more lines:
5yy
and so on. At the end, I want to paste all lines (in this case 8)
You can append to the named registers "a through "z, in which case you use the lowercase versions of the registers to overwrite/replace their contents and the uppercase version to append to their contents.
So you can use "a3yy (or 3"ayy) for the initial yank of 3 lines, then "A5yy (or 5"Ayy) to append the 5 next lines to that same register, which you can then put using "ap (or "Ap also works.)
See :help "a for details.
A neat trick to use when appending to named registers is that you can use the q command (which records macros) to empty the registers. For example, qaq will clear register "a. (It starts recording a macro into register "a, and then the second q ends the recording right away.) By clearing the register first, you can then use the uppercase register for appending on the first yank, which can be useful if you're using a repeated command such as a macro or a :g command to yank multiple lines.
