[pclmulqdq]

Postgres on modern hardware can likely service 100 million users unless you are doing something data intensive with them.

You can get a few hundred TB of flash in one box these days. You need to average over 1 MB of database data per user to get over 100 TB with only 100 million users. Even then, you can mostly just shard your DB.

[direwolf20]

What about throughput? How many times can postgres commit per second on NVMe flash?

[pclmulqdq]

You can do about 100k commits per second, but this also partly depends on the CPU you attach to it. It also varies with how complicated the queries are.

With 100 million DAU, you're often going to have problems with this rate unless you batch your commits. With 100 million user accounts (or MAU), you may be fine.