Why CONFIG_BT_CTLR_TX_BUFFERS can only be set to 19

[mfinmuch]

Hi,
I recently used ble multilink central, and the error "Tx Buffer Overflow" appeared after connecting for a while
I guess it’s because I have a problem when sending write_wtihout_rsp to enqueue. There are too many write_wtihout_rsp to send, but there is not enough enqueue buffer.
I want to increase the buffer of my transmission queue by increasing CONFIG_BT_CTLR_TX_BUFFERS or CONFIG_BT_L2CAP_TX_BUF_COUNT
But when I compile, the compiler says that my range cannot exceed 19
It is stated on the webpage that the maximum can only reach 19
web: https://docs.zephyrproject.org/latest/reference/kconfig/CONFIG_BT_CTLR_TX_BUFFERS.html?highlight=config_bt_ctlr_tx_buffers#cmdoption-arg-CONFIG_BT_CTLR_TX_BUFFERS
Why CONFIG_BT_CTLR_TX_BUFFERS can only be set to 19

In addition, is CONFIG_NET_BUF_TX_COUNT related to BLE?
It feels a bit similar to CONFIG_BT_CTLR_TX_BUFFERS

Thanks,
Poyi