Each generation costs one credit, so the arithmetic is straightforward: twenty credits is twenty images.
Packages start at $5 for 20 credits and the per-credit price improves at higher tiers, which is worth knowing if you expect to compare several garments or build a full photo set rather than testing a single item.
Because comparison is where this tool earns its keep, budget a few credits per decision rather than one. Seeing the same jacket in three colours, or a regular against an oversized cut, is what actually prevents a bad order, and that costs three credits rather than one.
Current package sizes and prices are listed in full on the pricing page.