$19, 13,$

Now, add $4$ with $19$

$19+4=23$

$19, 13, 23$

Next, add $9$ with $13$

$13+9=22$

$19, 13, 23, 22$

add $4$ with $23$

$23+4=27$

$19, 13, 23, 22, 27$

add $9$ with $22$

$22+9=31$

$19, 13, 23, 22, 27, 31$

add $4$ with $27$

$27+4=31$

$19, 13, 23, 22, 27, 31, 31,...$

Thus, next number is $31$