I developed a process recently in Accounts object and when I did a mass update of account records(around 600+ records), I got an error saying Too many SOQL queries: 101. This is due to the reason that process builder is not bulkified. Good news is that from winter 16 release onwards, the chances of getting this error would be reduced as Salesforce made process builder to handle bulkification.

Release notes link: http://releasenotes.docs.salesforce.com/en-us/winter16/release-notes/rn_forcecom_process_bulkification.htm#rn_forcecom_process_pb_bulkification.xml