Yes, that is the unknown variable. However, your costs can be steered pretty accurately in software startups through hiring and firing. This means you can easily track whether your sales are still hitting the targets you expected, and if not, how much reduction you can accept into on the costs side before you need to look into getting additional capital investments.

Edit: For existing businesses this metric is much more predictable by the way, but especially in B2B it might be obfuscated because the finance department does not know how much value has been provided for which there was not an invoice created for it yet.