You can reserve space only by using thick disks no other methods exist.
You should also keep in mind the following things:
- if you take snapshots, they occupy disk space as well. Initially delta disk size is 16Mb and it will grow when you write new data in VM.
- by default thin disks only grow in size. If you delete something inside guest OS thin vmdk disk on datastore won't shrink automatically. There are additional requirements to make this possible. Read this article for example Direct Guest OS UNMAP in vSphere 6.0 | Cody Hosterman and there was a bug in ESXi 6.5 https://kb.vmware.com/s/article/2150591
Also I don't know what storage array you use but check if you have thin or thick luns configured on your storage side.
If you have thin luns on storage keep an eye on them as well.