How much can I save and invest if I am an officer serving NS?

Depends on how much are you putting aside for NS allowance, on this investment. Maybe can start with $200 - 250, for Blue chip stocks, with regular dividend payouts. Do take note that share prices fluctuations are inevitable, so this is a long term strategy.

If not, can consider Singapore Savings Bonds, with min $500 fixed sum upfront, 1st year interest can go as much as 1.74% (Aug 2018 SSB), which is higher than high interest rate saving accounts, without additional criteria other than putting your money in. Dividend payout is automated every 6 months.

Hey, recently Seedly released an article which addresses your part of your question. Check out the article here - https://blog.seedly.sg/national-service-ns-how-much-can-one-save/

As for investing, I feel that unless you have a lot of free time during your NS, I don't think you should trade actively/day trade in the stock market. Can consider using a regular savings plan to invest into Nikko AM STI ETF through POSB investsaver as an alternative. If you feel that you're still up for it, you can consider trading using Standard Charatered as it charges the lowest trading fees of $10 as compared to other banks/brokerages. You can check out the comparison that Seedly did here - https://blog.seedly.sg/the-ultimate-cheatsheet-cheapest-stock-brokerage-in-singapore/

RSP, SSB if you wanna do a lump sum.

Since you are young, might wanna look into higher risk options though

It's always a good time to start investing, the earlier the better.

Your age is young, so I'd really recommend you invest with commitment rather than something thats so liquid you can back out of the last minute. Commitment breeds Wealth, and aggressive funds would make a lot more money than any other public instrument on the market. A good FA can also peg your investment plans to your objectives - because investing and saving money is extremely tedious without having a prize to look forward to.

You can always drop me a PM if you like. Investing is my speciality, and I made 32.7% in the market last year net of fees.

https://www.facebook.com/luke.ho.54