Unfortunately, there is not a definate answer to this question. It will depend on the types of pages being served. If they are all small static pages then apache could server tons of them as each would not take up much processing power or memory. If, on the other hand, the pages were massively complex dynamic pages the limit would be much lower.
For example is it ok for p 4 2.5 Ghz with 2 GB of RAM server to serve 150 connections ?
My initial response would be to say that this will be fine but again, it will depend on the content served.