Last time I had to fix one of the largest Chevereto based websites it was holding more than 2M images so that tells everybody that it can be achieved.
Is not that much the software which limits you here, but the machine or server architecture that you put into it. A dedicated machine doesn't tell you anything, only that is a real machine, not a virtual thing. I've seen some VPS servers working way better than some dedicated machines.
A dedicated server ensures that you will have the computing power to process a certain amount of load on an everyday basis, but that's all. You will want to leverage network by using CDN + external storage so the static requests won't struggle your main machine. 2M images will surely eat lots of traffic and network.
If you mean parallel uploads then yes, Chevereto allows you configure how many images will be uploaded at the same time from the same peer. The setting is referred as "Upload threads" and is capped to 5 because is a safe value, if you increase that number you could be exposed to DDoS. You can inject a higher value directly in the database.