Apache has a limit of how many concurrent connections it can handle. This limit is set with MaxClients. When the server reaches that number of simultaneous connections, any subsequent connection can't be handled until one of the existing connections is closed.
You also have a setting called MinSpareThreads, which tells the server how many threads should be started, waiting for a new connection. So let's say that your server is set like this:
Then, the server will try to always have 5 threads ready and waiting for a connection. But once you have 95 users connecting at the same time, you only have room for 5 more, so once the next client connects, the server will no longer be able to create any more spare threads - so the log message is telling you that it's getting close to the MaxClients setting.
It's always a good idea to understand why the server is reaching MaxClients. Its value should be high enough to serve a normal load + a reasonable spike in traffic, but low enough that even if someone tries to DOS your web server, apache will not use up all your memory and CPU by serving too many customers. To find out the optimal value you need to do some load testing.