After checking and searching, i found this ble_gatts_conn_cfg_t::hvn_tx_queue_size should be the key to set it. In my custom project (SD132 V5 + SDK14.1) the only setting related to this configuration is BLE_GATTS_HVN_TX_QUEUE_SIZE_DEFAULT 1 which is located in "ble_gatts.h".
I dont think i should change the default value in this header. In throughtput example, I couldnt find the codes that related of it.
So how to set the queue size up other than change it directly in the "ble_gatts.h"?
To increase the number of available buffers you need to increase NRF_SDH_BLE_GAP_EVENT_LENGTH. I.e. you cannot set the queue size directly, but have to configure the length of the connection event. You might also want to change NRF_SDH_BLE_GATT_MAX_MTU_SIZE. And maybe also consider using the connection event length extension option. But as long as you have enough memory, setting the event length to the maximum expected connection interval works, it will configure the device to have the maximum amount of tx buffers and send packets for the entire connection.
Thanks for your reply, I have been successfully increased my throughput according to your answer. Just for curiosity, where is the setting of connection event length extension? I cant find the setting. It would be great if the event can extend automatically.