I recently bough a Garmin eTrex 30x in preparation of a bigger adventure happening next year 🙂
I choose the eTrex 30x because it was an affordable, outdoor-proof, quite robust and long-lasting GPS device for outdoor Trekking. I need a device that has some hours runtime to be able to follow and record tracks.
And I wanted a device with a significant amount of storage, where I can put OSM (OpenStreetMap) maps on.
The procedure is documented sufficiently insufficiently, but fortunately there are enough informations available, so that you can put the steps together. I try to write here down, what I have done to bring multiple recent OpenStreetMap-maps to your device.
This post is based mostly on the OSM Wiki page for Garmin 
Put multiple OSM maps on your Garmin eTrex 30x
First you need a new SD-Card, where the maps will be located. I prefer to write directly to the SD-Card, but you can go as well through the USB-Mass Storage configured Garmin. Your choice 🙂
FAT32 formatted SD-Card works pretty well for me. I have not tested any other filesystem and since most of the pre-build OSM Maps are blow 4 GB, this should be fine. Yes, it's an outdated, crappy filesystem, but it does the job.
Download the prebuild-Garmin image files from [here]. You may have to look a bit around, but in the end you will find what you need.
Store the extracted gmapsupp.img files in the folder "Garmin" on the SD-Card. You may need to create this folder, if it does not exists yet.
To store multiple cards on the SD-Card just rename them, but keep the .img ending. Keeping the ending is an assumption by myself, I have not tried it out, but would not recommend removing it in order to prevent confusions.
 OSM Maps on Garmin - OSM Wiki