I'm an India national. I visited Singapore with a tourist visa and stayed there for one month (from Apr 23 - May 22nd). I want to visit Singapore again for 18 days in the month of July as tourist. If I apply for a tourist Visa within 45 days of my return from Singapore, will I get approval for my visa or is there any chances to get rejected?
You should be fine. Tourist visas get rejected if they think you are working illegally in Singapore, but if there's a gap of several months since your last visit and you only want to stay for 18 days, this should not be a problem.
Standard disclaimer: Nobody here has inside info, the only way to get a definitive answer is to apply and find out.