Well, it’s done by “rotating” the Hosts in the tabernacle. At every Sunday Mass, new hosts get consecrated. Some of those newly-consecrated Hosts are distributed, some of the already consecrated Hosts get distributed, and whatever is left I put in the tabernacle.
Sometimes the tabernacle has more, sometimes less, but in very general terms, on average, there are about as many Hosts in the tabernacle as a priest would distribute at an average Sunday Mass. There’s no set number. Bigger parishes tend to keep more in the tabernacle, smaller parishes keep less. The key phrase here is “very general terms.” It’s not as if priests calculate the annual average for a Sunday Mass and keep exactly that amount in the tabernacle at any given moment, so don’t read too much into the “average” part.
Occasionally, I make it a point to distribute all the Hosts in the tabernacle, just to make sure they don’t get too old. I usually “plan” this a little by making sure that the week prior, is a week where there’s “a little less” than average. By making sure I have less than I need in the tabernacle at the start, I know that at the end of Mass, the newly-consecrated ones will be the only ones that get placed in the tabernacle. Personally, I like to do this on the first Sunday of every month, just as a little “memory device.” Of course, it doesn’t always work out that way–so the schedule isn’t “carved in stone” or anything. I also consecrate a new large host for adoration/benediction on the first Sunday of every month for the same reason (that part I do keep very regular), so doing both at the same time likewise helps me to keep track.