How many connections per second can the leading web servers support (single server)?

It usually depends on how u handle the requests.. whether it is
handled by a front end with single threaded or multi threading
process.. also it matters on the cache size n all.. but if u look at
the most basic design where a server just accepts a request, looks for
the file on its disk n returns the file then the access time of a high
end SCSI disk is around 5 msec which restricts the server to handle
200 requests per sec at the most.

You've asked a question whose answer depends on all sorts of things
you have not specified: what hardware (CPU, speed, memory size), what
server configuration (you trade speed for robustness with different
Apache configs), what O/S (different versions of Linux have
significantly differences in TCP/IP stack code), where are the clients
(if they're local, then latency does not slow everything down nearly
as much).

You're probably simply asking the wrong question. Hard to guess what
your real intent is, but a question like "Can I service 10 WAN HTTP
requests per second for a 4KB static HTML file on such-and-such
hardware and such-and-such software configuration?" might be much more
likely to provide a meaningful answer.

Estimating web serving ability in advance for an arbitrary project
depends on an awful lot of variables.