As of this writing*, there are 6 Full Time (and funded) core developers working on PIVX. Beyond this though, there are many other part-time (and volunteer) developers assisting in other aspects and functions of the code from core-code, to wallets, and third-party integrations.
Also, PIVX is now one of only a handful of Cryptocurrencies that has an actual Academic/Professional Cryptographer onboard. Since PIVX is not just a clone or copy of other projects as the developers have completely customized PIVX’s own codebase, the community and developers saw the need and importance of bringing on an expert in cryptography to ensure that the codebase being developed (and continual ongoing work) maintained a continually high standard of security.
*Update as of February 3, 2018: We now have 6 Core Developers & 3 Jr. Devs!