Community version has no license costs, and there is no limitation for syncing a LDAP for 1500+ users. But there is no option for using it in cluster mode (only in Enterprise Edition), so the limitations are mainly operational and relative to the use cases of those 1500 users in "only one machine". You need to consider for example, if the main part of your users are casual or potential users with very ocassional use of the repository (let's say a 95%), or on the other side, you have significant percentage of continuously working users (concurrent user), for example (25% or higher). Also an estimated volumetry is needed..
I mean that it is not possible to scale the Alfresco application horizontally, with a cluster of several machines distributing load and running in parallel the Alfresco application. Only one machine. This would be suitable in high concurrency scenarios (many users).
To be precise, the cluster feature that Alfresco has implemented is not available with the Community Edition. Any cluster feature that was developed by a member of the Alfresco community could be used with Community Edition just the same. There is no legal / license restriction preventing that...