if you want to accept thousands of texts is this for revenue generation or to test a service? If it's to test a service you *could* use SIMs in gateways but each SIM is restricted to receiving around 1 SMS every 2 seconds. If you need to receive high volumes inbound on a single number then you'll need to go for a network provisioned "long number". This looks like a normal mobile number but is "virtual" within the SMSC of the network. The long number can receive way quicker than a normal SIM. In the same way you could get your own shortcode or shared shortcode with keyword, however the cost to send to the shortcode will be different than a long number.
What country do you want to send to / receive in?