How many keys.......

Should work on a keyboard at anyone time.

I play FPS games and ive been having problems with keys not working when pressed. What ive put this down to is pressing more than 2 keys at once.

I.E If i am pressing Up arrow and right arrow at the same time for movement, then i cannot use any other key, to select a weapon/use an item?

I have a USB mouse plugged into a USB 3 port and i am using a (Razor Lycosa keyboard)

I have never had this problem on PS2 keyboards in the past.

 

It varies with each keyboard and is down to their anti-ghosting. Some, e.g. the MS Sidewinder X4 (which I have), can do 26 keys at once whereas others can only manage a couple.

 

Most keyboards are '2 key rollover' which means they only read two keys at a time.
'N key rollover' means all keys are read individually but as this costs more to manufacture it's not common.
As Ash says, decent gaming keyboards should support what you need even if they aren't true N key rollover.
Have a look at Peripherals » Keyboards and Mice » Gaming Keyboards - Overclockers UK

 

That's not quite correct.

For most keyboards It depends on the exact keys you're using, almost all keyboards will guarantee any two keys will work together but whether three, four, five or six will work together depends on the specific keys and how the keyboard is wired up.

So if you're not exaggerating about no other keys not working then it's not this issue, you would be able to use most of the other keys on the keyboard with only a few being blocked.

A small number of gaming keyboards do offer higher guaranteed minimum numbers of keys for all or part of the keyboard, your Lycosa does this for the 'gaming cluster' which I believe they define as WASD and surrounding keys.

It's nothing to do with USB, USB can limit keyboards to six keys at once but that's generally not relevant.

p.s. On terminology. For a long time this has been a very niche feature and hasn't really had a proper name, the vauge 'N key rollover' seems to have had the most popularity during that time. With it being picked up by gaming keyboard makers lately there's been some confusion over what to call it, some of them have slightly unwisely gone for 'anti-ghosting' which sounds cool but is technically wrong. Ghost keys are those that come up when you didn't press them, which will happen with the standard keyboard design unless they're specifically ignored so any keyboard can claim to have anti-ghosting technology whether or not it guarantees a higher minimum of simultaneous key presses that normal. Caveat Emptor!

 

You see the gaming keyboards always offering the ability to smash the keys without it losing anything!

I've been looking at getting the corsair k90 but like a lot of the decent keyboards, its a costly affair!