What are the limits to the number of merge codes which can be assigned/populated?
There are 10 custom fields for Invites, I believe. Is that the limit?
If one has more fields or queries to insert, is there a limit to the number of url variable merge codes which can be created and assigned?
Are there any other limits I need to be aware of in designing the survey?
Can the API handle all of these?
SurveyGizmo may or may not have a limit on URL variables, but there is a practical limit in that the full URL should not be longer than about 2000 characters. See:
I do not believe there is a limit to number of merge codes used. I have some Salesforce actions that query using 10+ variables that I insert via merge codes. Regarding URL variables, again, you can have multiple variables. The article below does not state there is a limit and provides an example of one with multiple URL variables.
Regarding custom fields for invites it sounds like you should create an Email List where you can create as many custom fields as you want (if the default fields + 10 custom fields are not enough). For Email Lists, there are 20 “Standard Contact Fields” and you can add “Contact Custom Fields” from there. You can read more on that here:
We’ve never had an issue with the API and merge codes