How to install a libc6 version >= 2.29?

To my knowledge, libc6 2.29 is the latest version. And it is available for Ubuntu 19.04 (link).

If you want to install libc6 amd64, open this link to download it, then open the download folder in the terminal and run sudo dpkg -i libc6_2.29-0ubuntu2_amd64.deb

If you want to install libc6 i386, you can download it from this link, then open the download folder in the terminal and run sudo dpkg -i libc6_2.29-0ubuntu2_i386.deb

---

EDIT 06/01/2020:
As per now, libc6_2.29 does not exist anymore. However libc6_2.30 and lib6_2.31 are available via this link (2.31 is the official version for Ubuntu 20.04).
If you want to download a different version than the official one supported by your Ubuntu release (i.e 2.27 for 18.04, 2.30 for Ubuntu 19.10 ...), check this link
Download the version that suits you, then open the download folder and run sudo dpkg -i <the_file_name>.

Don't do it. Here I'll leave an image of how dependent are your software packages on libc6 compiled for that specific library!!! https://sites.google.com/view/dependency-netwok/ There you'll see a star shaped network and the node at the center (green colored) represents libc6.

I suggest two possible solutions to your problem:

1. Add a compatible repository

Since it is r-base-core what you want, change the repository to a compatible one for your specific distribution.

for example: deb https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/

In your /etc/apt/sources.list file. By using you will be automatically be redirected to a nearby CRAN mirror. See https://cran.r-project.org/mirrors.html for the current list of CRAN mirrors.

sudo apt-get update

sudo apt-get install r-base

sudo apt-get install r-recommended

2. Upgrade

Another solution would be to upgrade your entire distribution to a more recent and compatible with what you want.