Consider adding a 1/2- or 3/4-bath, particularly if both of your existing bathrooms already have bathtubs. We have a separate guest studio building that is 400 sqft, and it has an entirely adequate 3/4 bathroom with a 32" X 56" shower that we carved out of a 6' X 7' space, and it meets all code requirements.

If one of the two baths is an en suite devoted to one bedroom, I would say it's worth adding at least a half bath. If you have the space, go for a full bath. An extra toilet is nice and even just a small standing shower can help make your home run more smoothly when all of the bedrooms are full. I don't know the exact trade-off on resale. There are a lot of factors that will determine that, including the finishes you choose in the bathroom, the cost of the work in your area, and the overall impact the bathroom has on how your home flows and fits together. A bad reno can be worse than just leaving the house as is, so the question of "will it help resale?" can be really tough to answer without managing the project every step of the way. Example: Removing the master bedroom's walk in closet to add a bathroom or adding a bathroom that opens directly into your once fabulous kitchen.... Probably won't have the desired trade-off. Those are some decently obvious pitfalls, but there are many, many other less obvious ones that would only come up on a case-by-case basis.

That said, I've been doing a lot of research on bedroom:bathroom ratio. It isn't an exact science, and I didn't crunch hard data, but anecdotally, this is the estimate I come up with. If n is the number of bedrooms....

The number of toilets should be at least n-2.

... 4-2 = a 4 bedroom home should have at least 2 toilets.

The number of toilets in the home gains little above n+1.

... 4+1 = a 4 bedroom home gains little value from having more than 5 toilets.

So, having between 2 and 5 toilets is good for a 4 bedroom home. The home loses sell-ability with only 1 bathroom. The resale increase diminishes for more than 5.

The number of showers in the home should be at least n-2.

... 4-2 = a 4 bedroom home should have at least 2 showers.

The number of showers in the home gains little above n.

... 4 = a 4 bedroom home gains little value from having more than 4 showers.

So, having between 2 and 4 showers is good for a 4 bedroom home.

All homes should have at least one bathtub. If there is only one bathtub in a home that has at least 3 bedrooms, that bathtub should be kid friendly (most people buying 3+ bedroom homes are parents or people planning to be parents). I haven't figured out a rule of thumb for when it becomes necessary for there to be a second bathtub, but at some point, more bathtubs become essential to provide the luxury that fits the home.

Hope this helps.

For future or potential resale, not necessarily. If you need more bathrooms then it is a different story.

I would add a 1/2 bath

There are different answers to your question OP depending upon the neighborhood itself. In some neighborhoods the expectation would be 1 full bath for every bedroom + a 1/2 bath for guests so they don't have to go into the bedroom area. If you have fewer baths in this type of neighborhood, your home's value will take a hit. How much? Depends on the neighborhood. FYI: I am using the term "full bath" loosely. You could have either a 3/4 bath (3 piece bath = w/c + vanity + shower) or a full bath (4 piece bath).

Other neighborhoods would work fine with 2 baths for the entire home. What does your neighborhood consider adequate? If you want to be more than adequate, add a 3/4 bath.

The incremental value added also depends on the neighborhood and the time you sell.

The real answer is - what do you need to function well?

What would make it worth it to you to have an extra 1/2 or 3/4 or full bath?

Budget and space constraints play into a large part of this answer.