There aren't many options when it comes to setting up the Data Stream for Synchronised Data Extensions unfortunately, like setting a specific row limit.
The least you can do is configure the Synchronisation to either:
1) Synchronise only records created after X date. With this you could limit it to be created from October 2018 for an example. This is not going to get you a specific amount, but more of a ballpark which you can check first with SF reports.
2) Synchronise only records with a specific field value. You can use some existing field to determine which records you synchronise over, or alternatively you can create a new field (example - checkbox "MC-Synch") and mass-update the exact amount of records you would like to get synchronised over.
Here's where you can pick one of the options: