You say the issue your boss has is that too many employees are taking off at once, so why not help him to address the issue.

Try introducing a shared calendar or somewhere your team can document its scheduled time off. Having this, you should be able to come up with some general guidelines about how many team members are needed at any given time. This also gives your boss a place to mark "critical" time periods around deadlines where more team members are needed.

Having a concrete policy should remove any confusion around the issue, just be sure the rules/guidelines are reasonable. If you find your boss resistant to such ideas, it could be a sign of a deeper problem. Do what you can to get to the root of the issue, but be prepared to move on if you feel you're being taken advantage of.

You say the issue your boss has is that too many employees are taking off at once, so why not help him to address the issue.

Try introducing a shared calendar or somewhere your team can document its scheduled time off. Having this, you should be able to come up with some general guidelines about how many team members are needed at any given time. 

This also gives your boss a place to mark "critical" time periods around deadlines where more team members are needed. Having these critical periods documented will force him to make decisions about what is important, where left unchecked he may be likely to see any time period as critical (there will always be deadlines!).

Having a concrete policy should remove any confusion around the issue, just be sure the rules/guidelines are reasonable. If you find your boss resistant to such ideas, you may find that he's not been truthful to you. This is good to know. Do what you can to get to the root of the issue, but be prepared to move on if you feel you're being taken advantage of.