I’m intending to invest in DCA thru FSMone for SPY500. How much should i put?

It depends on your savings and needs. If you need the money within the next 3-5 years, then invest lesser and save more money. If not, you can increase the amount you invest.

I think $300/month is a decent amount to get started with. It's always good to enter the market with an amount that you are comfortable with, gain a better understanding of how the market works before deciding whether to increase that amount when you feel it's appropriate.