I would work your way up to 3 pills per day or about 60 billion CFU per day. Its up to you of course and what your budget can handle; the more you use the more it’ll cost.
It is a good idea to rotate probiotic brands over time but if you have a lack of choices there, this isn’t the end of the world.
As far as when to take probiotics; I take mine with meals personally and it can be taken with or without meals. Its up to you on when to take them.