Saving a few seeds from your tomato crop is a lot simpler and easy than you might think. The first thing to remember is that most tomato varieties will not cross with each other. This means that if you grow several different varieties in the sae space you can, generally, save seeds from all of them and they will remain true to type. This is because nearly all indeterminate (Climbing) varieties have flowers with inserted stigma. this means that a visiting bees is unable to pass pollen into the flower which is completely self fertile. As soon as the pollen within the flower drops onto the stigma the tomato fertilises itself. There are exceptions however. Many older and traditional potato-leaf varieties as well as bush or determinate ones have exerted stigma which means that a visiting bee can transfer pollen from one plant to another. The good news is that you can grow several different varieties of tomatoes together and they will not cross if they have flowers with inserted stigma. If you are growing a variety with exerted stigma you will need to isolate it from other tomato varieties. If the flower looks like this then it has inserted stigma.
You can learn more about tomato flowers here https://blog.seedsavers.org/blog/exploring-tomato-flower-structure