Registration becomes compulsory when your taxable supplies exceed R1 million in any consecutive 12-month period, or when you sign a contract that will take you over R1 million in the next 12 months. Once you cross the line you have 21 business days to register.
The phrase doing the work there is "any consecutive 12-month period". It is a rolling window, not your financial year, and that catches people out constantly. A business growing through the middle of its year can cross R1 million in October and be obliged to register then, months before anyone looks at annual figures in February.
Below R1 million you can register voluntarily once you have exceeded R50,000 in the past 12 months. Whether you should depends on who your customers are. If they are VAT-registered businesses, they claim back whatever you charge them, so registering costs them nothing and lets you recover the VAT on your own expenses. If you sell to the public, adding 15% either raises your prices or eats your margin.
Registering also brings obligations: VAT201 returns every one or two months depending on your category, valid tax invoices for every input claim, and records kept for five years. That admin is a real cost and belongs in the decision.