It depends on how many players and plugins you are wanting on your server at any given time.
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Why would you make a minecraft server to play on it urself? It wastes RAM and time.
1. Your computer doesn't have enough ram. 2. There is too many people on your server. OR 3. Your server doesn't have enough ram. :)
You may need to check if you have no programs running on conflicting ports, and have enough RAM allocated to the server.
You only need one, but you would need lots of DDR3 (RAM), maybe 2+ GB.
Typically, Minecraft usually uses 300MB + RAM - Usually about 600MB, but it is automatically allocated up to 900MB RAM (if your computer can handle that). Basically, the more RAM you have, the better Minecraft will run.