Here are some interesting numbers involving Baltimore's United Cable system:
* By year's end, the company hopes to reach or at least come close to the 100,000-subscriber level. Yet by the end of 1991, that level is projected to grow just to 110,000. By the time the system reaches "maturity" in five years, the projection is for only 130,000 subscribers. (There are a little less than 300,000 residences in the city.)
* Now, a little more than one in every three residences gaining access to cable subscribes. The company hopes to improve that "penetration level" to 38 percent next year. Growth to 45 percent is the most optimistic goal at system maturation.
Officials say subscription rates may never be better than 15 to 18 percent in some poorer sections, while other areas might subscribe at a rate of 60 percent or better.