When starting the server it starts usage at around 500Mb~ and hovers around there for a few minutes. With no players online, it climbs until it reaches around 10Gb then drops down back too around 1Gb. Does the same with players online.
Provided that this is about Minecraft: Java Edition, if you give the server up to 10-ish gigabytes of RAM allocation, that's normal; As far as I know, Java's "RAM garbage collection" kicks in to clear unused RAM away when it's about to run out of RAM.
The more RAM that you allocate to the server, the less often it has to do this, but the bigger a lag spike will occur when it does. (Conversely, if it has less RAM allocated, it will do this more often, but with smaller lag spikes.)
This also applies for players/clients, not only servers. (The RAM allocation and usage are not the same between players/servers, but it's still nice to know.)