I have table users which has millions of data

users have colums like id, competed_date, thirdparty_id etc etc

thirdparty_id is new column i have to update for all users

proces to find thirdparty_id

for each user there is corresponding order table from which we have to fetch latest order
from which we will get one value based on it we can calculate some rate

that rate we can search in another table thirdpary from there we will get thirdparty_id

i have did everything for individual users its working fine

now my question is how to execute this for millions of users?

i am using laravel

the process is fetch all users having thirdparty_id null
and call formula function to find id and update

but fetch all means millions of data in single query?
so if i am giving limit whats maximum limit i can give?

what are another options to execute?