Shopify Basic plan does not limit by number of visitors and sales amount.
The difference b/n Basic and “Shopify” is that you will be charged higher sales commissions on the sales you make.
And if you expect increased amount of sales, it might be wiser to switch to the “Shopify” plan.
I think the threshold is at around $3,500 in sales - above that amount, it would be wiser to switch to the next plan.