This chart shows the max_connections limit, set by AWS, at various MySQL RDS instance sizes.
AWS sets the MySQL max_connections system variable according to the size of the RDS instance. max_connections is the maximum permitted number of simultaneous or concurrent client connections. A “Too many connections” error message may be displayed if this is reached.
If an application implements long-running queries or creates several concurrent connections from the web or app side, the number of connections is just as important as vCPU cores, memory level or storage IO.