How many txid gets added to mempool per second? I am trying to get the txid using json/rpc and trying to find out how many tx_id gets added to memory pool ? 

i think it comes down to how many bitcoin transaction are possible per second ?

is there any fixed number ? or maximum number ?
On a long term average the number of transactions is limited to the number of transactions that can enter the blockchain-- so roughly 7tx/s before segwit and 14tx/s after segwit activates.

But on a short term basis while the mempool fills or with transactions being replaced the rate can be as fast as your node can process transactions.  On reasonably fast hardware this can easily hit a few thousand per second.

But you won't normally see rates like this in practice unless someone attacks you or someone does something strange, especially since most nodes limit their instantaneous relay rate to a small multiple of the long term average transaction rate.  When a new node starts up it only gets newly arrived transactions, though a misguided or malicious peer to could advertise every transaction they know to you as soon as you come online in order to hit that peak rate (at least until your mempool fills).