Can Commerce be used across different contexts?

Yes, your shop can be spread across different contexts. They'd all be served from the same MODX installation (one manager) and a single Commerce dashboard. The product records would be shared across the MODX installations, but your catalog could be vastly different per context.

It's possible to use different themes per context, as well as setting up different cart/checkout pages with context-specific settings. Note that you will need to create the cart/checkout pages in each context if you want to use context-specific settings in orders (e.g. cultureKey/locale) or multiple currencies. 

It's also possible to set a different currency per context (which requires context-specific cart and checkout pages), in which case you should also set up currency-specific product prices.

Only one Commerce license is needed for multi-context setups, as long as you only use one primary domain in the manager.