I rebuilt the Manufacturing Mobile app from scratch in React. If I host it myself and use an Ouath1 token for requests can use just one license?

you can use 1 license for your api calls. We do this and hit numerous restlet, rest services etc. The prob you might run into is depending on how many users you have simultaneously and netsuite processor licenses (i forget the term) you have some calls might come back as 'too many requests' if some users have netsuite license you can can create oauth tokens for them specifically but will still bump into your max simultaneous processor limit. this is at least what we have experienced

Thanks. If I ask them will they tell me no to try to squeeze my company?

our contract listed 5 concurrent API users and SuiteConnect+

Not worried about overloads. I'm talking max 20 users. Can't imagine more than 2-3 simultaneous calls.