Mendel used over 28,000 pea plants in his experiment. How does this large
sample size make his results more reliable?

Experiments in general are more reliable when a greater sample size is used. When using a small sample size, a mistake or fluke can greatly effect the outcome of the experiment. With a large sample size, there is more data to understand averages and deviations to the average data. The average will likely be more accurate with more data, which would make his sample size more reliable.