Instead of using manual updates, consider using Entities which let you share data between forms. You can think of an Entity List as a CSV attachment that's shared between forms and can be updated by form submissions.
To generate sequential ids, you could use a household Entity List that keeps track of the highest participant ID assigned within that household as @joybindroo suggests. Submissions can then automatically update that highest ID. You can use pulldata in the same way as described.
Another option would be to use a participant Entity List that keeps track of each participant, the household that they're in, and their number in the household. You can then use the max function to get the highest participant id used so far and use it as the basis for the next participant id. This would be a good approach if you also want to use participant information such as their name in future follow-up forms.
Let us know if it would be helpful to see a worked example for one or both options.
Currently, Entity updates require getting a form update from the server but soon they will happen offline as well. That means you could generate sequential IDs for two participants in the same household that you visit one after the other without connecting to the server in between.