Yes, only one email activity per lead per day synced and no, no way to choose only some to sync their activity to Salesforce. You'll probably want to use Sales Insight over Activity sync to more completely track Marketo email activity.
As Steven has mentioned there are limitations to the activity sync feature since its deemed an old feature and the recommendation is to use Sales Insight instead.
You can find more information about the feature and its limitations such as it only syncing the first activity every 24hrs in the docs Customize Activities Sync - Marketo Docs - Product Documentation
There are a number of threads discussing the pitfalls of this feature and why it would be more useful to use sales insight, search the forums like this one Customize Activities Sync in Marketo