This varies from broker to broker. Most platform providers have very low minimum deposit requirements in order to remain competitive.
However, just because you can open a trading account with as little as $50, doesn't mean you should
You should always have enough capital in your account to cover margin and starting with a balance too low and opening trades that are too large for the account size is a quick way to lose money.
Take some time to consider how much available capital you have to dedicate to trading and always remember, any capital allocated to trading is always at risk!