Thank you all for the answers!
Main goal of iRule is to monitor number of users logged into an appication and, if number exceed a limit, to block access for new users into application.
iRule is working fine for me, but I have been requested to send periodically number of active users so we can set thresholds correctly and this was something I did not know how to do.
I already solved problem thanks to CA_Valli idea altough I used tables in my iRule I did not think it can have such a simple solution (may not work correctly for low number of requests).
if { [table lookup -notouch "timer_check"] != "Y" } {
log local0. "Number of users: [table keys -subtable $tbl_conns -count]"
table set "timer_check" "Y" 10
}
I set up value in table with timeout 10 and everytime value timeouted I send log message and set up value again. Worked correctly for me with about 50 requests per second, not tested for higher number yet.