Depending on how close to or far from other tomatoes they're grown. Toms cross very easily and seeds are not always true. It's why there's so many varieties.
I used to make sure I separated varieties when I first started saving seed. I have always read they would cross easily.
At that time I was a member of an internet forum that involved gardening. It was full of tomato growers in home and market gardens and also seedsmen (to sell comercial seed). They convinced me that while tomatoes can cross they do not cross as easily as most people think without human assistance.
I grow indeterminates in the ground. They get tall. It is easier for me to trellis tomatoes in a row. I didn't like separating them.
So I tried it. I planted Black Krim and Big Beef in the same row. I save Black Krim seeds.
I purchase Big Beef because it is a hybrid resistant to nematodes.
I'd have to look at my notes but I have been saving the BK seeds for a very long time with no evidence of crossing in growth habit, tomato color or taste. Even the baby green tomatoes look very different from each other.
Maybe because these 2 tomatoes look and taste so different makes me confident they have never crossed. BK is flattened and puckered at the stem, dark and twangy, juicy. BB is round, red, sweet, meaty.
In the past I have had bells and cayenne cross. Now I plant bells in the big garden and cayenne in the bed by my greenhouse. There is probably 1000 ft and woods between the 2 gardens.
Just my experience with these 2 types.