The allowed votes are based on your community ‘Trust Level’ which is determined by your activity in the community. 
| TL | Description | Votes |
|---|---|---|
| TL0 | New User | 2 |
| TL1 | Basic User | 4 |
| TL2 | Member | 6 |
| TL3 | Regular | 8 |
| TL4 | Leader | 10 |
One of the principles that the Discourse team mentioned with the voting plugin is having a limited number of votes to drive prioritization of where votes are cast. As @Bry mentioned, you can always unvote for something if you want to move the vote over to something more important to you.
Additionally, whenever a feature request is closed, those votes are returned to you.
You can also see it in the Activity section of your community profile: