What is a good amount of units to buy from a company? I am thinking of setting aside $500 per month to invest in stocks, what are the best options?

Think better to invest every 3months, so total available sum be (500+200)*3 = $2,100. $2000 for investment, $100 is way more than enuf for transaction & comm fees (even 1% of $2000 is $20).

No good amount of units per counter, only X% weightage per counter of the portfolio, to ensure adequate diversification.

Its best to keep your commission fees as low as possible. If you were to use DBS Vickers Cashupfront (the one i currently use) there will be a fee of $10 per trade. I like to keep my fees to be <1% of the total investment so minimally invest at least $1000 per stock counter will be good. Of course if you can invest more it will be better. $500 is sub optimal and you are incurring too much fees. Even before a gain you already incurred a 2.2% loss. This is one of my first investing mistake when I first started, not knowing the significance of trading fees. I wanted to make a 90-10 stock bond distribution and bought a really small position of bonds which led to a bad average price. I learnt my mistake and now always invest more to cover the trading fee of 10% and keep it below 1% of the overall investment

I would recommend investing 50% in a market portfolio like S&P500 ETF as it historically yields 10% return and has a general upward trend. Another 50% can be other individual stocks that you like and see a potential of 10% return. Diversification is key. The most vital thing now is do not rush into buying popular stocks now in the COVID period! For example, airline stocks just because they're cheap. ​​​

It will be good to keep your transaction charges to less than 1%. And do note that the US market is more volatile than SG market.