In this topic is a solution:
http://forums.alliedmods.net/showthread.php?t=45016
But it's not working for me. I got 22 slots on my server, 2 reserved and hiden (amx_reservation 2, amx_hideslots 2, amx_visibleadminslot 1, i tryed amx_visibleadminslot 0, but still nothing). If server is full (20 slots) and if admin joins the server then he can't join because server is full (reserved slots aren't working). So what settings i have to put in that plugin to fix that bug? And this bug isn't fixed even in the newest AMX Mod X version (1.76d). But that bug has been reported in year 2006. So AMX Mod X programmers hasn't seen that bug.