Are you concerned about Marketo API limit or SFDC Limit? It sounds like that you are concerned about Marketo limit. In your case using the "Sync Lead to SFDC" flow action in your Smart Campaign or the every 5 mins Marketo->SFDC sync do not count towards Marketo API Limit since they are both internal Marketo processes (Not considered as accessing Marketo API). However, it will increase your SFDC API calls (see article below).
Also, an API call to Marketo or SFDC doesn't count the number of fields being updated. It is the number of lead objects that matter. So updating 1 lead with 100 fields will be considered 1 API call in Marketo.
The 2 articles below provide further explanation on both Marketo->SFDC API limit and Any System -> Marketo API Limit
http://community.marketo.com/MarketoArticle?id=kA050000000Kyr6CAChttp://community.marketo.com/MarketoDiscussionDetail?id=90650000000PVi7AAGLet me know if this helps.