There are a lot of people installing Sentinel right now, which is likely disrupting the masternode side of the network to a noticeable degree. I expect these disturbances to die down within the coming one or two weeks, with most of it likely settling within the next few days.
On explorerz.top, at the time of this writing, I can see that there are 556 masternodes overall and 108 of them online. Be aware that the ones that aren't counted as online are either flagged as "EXPIRED" (only two), "NEW_START_REQUIRED" or "WATCHDOG_EXPIRED" (fixed by installing Sentinel). A good bunch of them is very likely running and "online" from a software perspective.