That's up to you...less refund at tax time....true, but that means more in you regular paycheck
...but if you put in too many allowances, you end up owing at tax time, with possible underpayment penalties tacked on.
Things to watch out for:
1) You cannot claim girlfriend or the 7 year old as dependents unless they lived with you for all of 2018 and you provided over half their support.
2) You can Not claim GF or 7-yo as dependents at all if GF has a job and earns over ~$4,100 (we've been arguing the exact amount...but should know by December). even if you did provide more than 1/2 of their support for 2018.
3) you can claim the mutual child as a dependent for 2018, or let GF claim if she has a job and earns over ~$4,100 (again, exact cutoff not yet clear)
4) If you get married anytime in 2018, then both children can be claimed as dependents, and GF, (now wife) as spouse ....but watch out for any child claim tax agreements between spouse and former husband for the 7yo. That can get messy...don't file for 2018 until all that is worked out next Feb or March when you actually attempt to file your 2018 taxes