I wash my bras after every time I wear that bra because I just don't like wearing anything that hasn't been washed previously.
However you can wash your bra less often than every time you wear the bra.
Most people wash their bras every 2 to 3 wears which is usually good enough.
So you can wear the same bra for up to 3 times before throwing it in the washing machine and washing it however you can wash the bra more often if you'd like too.