In a "just counters" scenario you can't imagine how much data a single Redis server can hold! It is sensible to understand when it is a good idea to use a database that holds data in memory, as there are obvious limits (that are are trying to break in a comfortable way with Redis Cluster instead of forcing client-side sharding), but on the other hand it is good to do some math before being worried.

Example: Redis can hold 1 million counters, every counter stored in a different key, for every 200 MB of memory. This means 5 million counters per gigabyte. Clearly you can have a lot of counters. But you can make this figure a lot higher if you aggregate counters using hashes: with this trick when applicable the above figure will be five times bigger.

Can you hint how many distinct counter increments can i expect to have per second with Redis?

100k increments per second in an entry level Linux box. 50k increments in a decent EC2 instance. 150k - 180k increments per second in a fast modern Linux box.