Evening,
HA has settings to help you automatically determine this answer. A few things to consider:
1. It looks like all your vm's are using around 32 GB's of RAM at this time. Given that your hosts have a total of 32GB's of ram it's possible to run all your vm's on a single node. Which means if you loose a single node you are still in business.
2. Memory can be over commited in vmware a lot... bear in mind when you over commit memory you create slow down's across the board. So yes you can add vm's but if one host fails or you need to upgrade all your vm's may slow down. How much of a slow down depends on your vm's needs.
3. HA has settings to prevent power on of new virtual machines if the user set threshold is crossed so you may not be able to power on more vm's without adjusting this setting.
So end result you can add more vm's but personally if this is a production environment with business critical app's it's time to buy another server.
Please let me know if you have any questions.