If you want it to last for many shaves you can do one thing.
Use a plastic salad bowl to make the foam.
You make the foam with a cheap Synthetic brush (they are more resistant than natural hair).
The trick is not to clean the Brush at the end of the shave, you just gently squeeze the excess lather and put it in the salad bowl.
And you leave the salad bowl with the remains of soap, foam, and the Brush without cleaning.
The next day you just have to add a few drops of water and you make the foam without adding soap.
With this trick, the same amount of soap lasts twice or three times as many shaves.
You only add soap one day yes, one day no.