Using a modern UEFI based system requires dividing the disk into smaller sections.
Required sections is a EFI partition and the system's root partition.
You need to create an area for swap too. Swap space is a temporary storage location for memory shared by applications. If you want to use hibernation you need to create a swap partition of the same size as your RAM. If you system uses a separate graphics adapter you need to add the adapters memomory size.
A Manjaro system can run with a root partition of less than 10G but doing so will require strict dicipline and maintenance to avoid your system root from running full. A root partition of 20-32G is more than adequate.
Many experienced users like to setup partitions for different purposes. The only extra I will recommend here is a partition to hold the home folders.
This makes a total of 4 (four) partitions: