What is the amount I can contribute to SRS? Is there a limit, and if so, how is this limit determined?

You may contribute any amount to your SRS account up to your SRS contribution cap.
Your SRS contribution cap is determined by multiplying the appropriate SRS contribution rate by an absolute income base.
SRS contribution rate
The SRS contribution rate for Singaporeans and Singapore Permanent Residents (SPRs) is 15% while the SRS contribution rate for foreigners is 35% in view of the fact that they do not enjoy tax relief from CPF contributions. If you are a foreigner, you are required to give a written declaration of your foreign status for the calculation of your SRS contribution cap. You must make such a declaration on an annual basis for the SRS operator to calculate your SRS contribution cap.
Income base
From 1 Jan 2016, the absolute income base is $102,000.
Contribution cap
This works out to a contribution cap of $15,300 per annum for Singaporeans and SPRs, and $35,700 for foreigners.