This question would belong to a branch of astrology called "predictive astrology." With predictive astrology, questions about many different topics into the future can be answered.
In western astrology, predicting the actual numerical number of children gets rather tricky. While the 5th House rules children, in general, the 5th House isn't always used for specifics about individual children. For example, there are some configurations seen in the natal chart, typically with saturn, that will deny or delay children. I hesitate to say this, because everyone with saturn in the 5th house will NOT be denied children...the entire chart must be taken into consideration. In general, aspects with mercury will show a small number of children, less than 3. Aspects with Jupiter may show 3 or more children. (Although this is relative...if your culture or family usually has 5 children as a "small" family, then mercury would be used, for example, and Jupiter if more than 5).
The 5th House is always used for children, in general. In addition, the 5th House is used for the first child in both the male's and female's chart. This should be verified with a synastry or composite chart between the male and female.
The couple's second child is represented by each parent's 7th house and composite/synastry to verify, the third child is represented by the 9th house, and so on. Determining the number, then, becomes a matter of what is not in the chart. If there are good aspects and indicators for a first and second child, but none for a third child, it can be concluded that there would be only two children, for example.
So, to answer your question, yes, astrology can indicate the number of children. But, no, it isn't always the 5th House. Anticipating your next question of how to determine this, I can only say that each chart, and husband/wife composite or synastry chart must be looked at individually paying attention to relevant aspects, planets, and houses.