Don’t put all your eggs in one basket
Don’t concentrate all your efforts and resources in one area as you will have no alternative if it fails.
e.g. My sister has been offered the possibility of a modelling job. She is now considering giving up her degree course but I have warned her not to put all her eggs in one basket.
The idiom uses the idea that if you gather hens’ eggs into one basket and you drop it, you will lose them all.