I know this is a horrible, generalized question with no good answer, and I apologize ahead of time, but I wonder if someone could take a stab at a very broad estimate. 

Let's say you have a dedicated MySQL server running on about $1K worth of modern hardware.

Let's say the average user makes 20 read requests and 5 write requests per minute -- all straightforward queries, no joins; mostly along the lines of 'select this row by UUID' out of an indexed table of ~10,000,000 rows.

Very, very, very, very roughly how many concurrent users would you expect a server like that to handle before you're 'pushing it.'