4 units are free. If i want to deploy 6 units，How much do I have to pay each year？
We don’t have a published price list for self-hosted installations. Email info@memsql.com and they’ll help you out with pricing. The prices are quite reasonable compared to other enterprise-class database products.
Also, even at the 4 units level, if you want to get support, you need a paid license, and that can be well worth it for many users.
Here’s our pricing for Helios, our cloud platform-as-a-service: SingleStoreDB Cloud - Pricing