I understand that Rochel saw she was not bearing children so she gave her maidservant Bilhah to Yakov but Leah was able to bear children. True the verse says she stopped from bearing children but what was the rush to more children? Shouldn't she have waited until her sister had a child first? What was the need to give Zilpah to Yakov specifically at that time?
The Ramban (B'reishis 30:9) comments that this was a means for Leah to ensure that the majority of Ya'akov's destined twelve sons would emanate from her or from her maidservant who was under her domain:
ותרא לאה כי עמדה מלדת - לא ידעתי מה המעשה הזה ללאה, ולמה נתנה שפחתה לבעלה, והיא לא היתה עקרה שתבנה ממנה, ואין דרך הנשים להרבות נשים לבעליהן. אבל נצטרך לומר כי היו נביאות (ב"ר עב ו), יודעות שעתיד יעקב להעמיד י"ב שבטים, ורצתה שיהיו לו רוב הבנים ממנה או משפחתה שהיא ברשותה, ולא תתגבר אחותה עליה בבנים, ולכך אמרה נתן אלהים שכרי אשר נתתי שפחתי לאישי (ראה זוהר ויצא קנז ב), וכן יעקב בעבור זה שמע אליה, שיעמיד בנים רבים, כי ידע כן כדברי רבותינו (ב"ר סח יא): ויתכן כי מדעתם שנתנה הארץ לזרעם, ואברהם ויצחק לא הרבו בנים, היה חפץ יעקב בנשים רבות להרבות זרעו לנחול את הארץ, כי דור רביעי ישובו הנה, ולכן רצתה ליתן לו שפחתה שלא ישא נכרית
Below is my hasty translation:
"And Leah saw that she paused from giving birth" - I don't know what motivated Leah to take this action, and why she gave her maidservant to her husband, when she was not barren that she needed her line to be built up through [Zilpah]. And it is not the way of wives to multiply other wives for their husbands. But we must say that [Rachel and Leah] were prophetesses (B'reishis Rabba 72:6) who knew that Ya'akov was destined to found twelve tribes, and [Leah] wanted most of the children to be from her or from her family that was under her domain, that her sister not surpass her in [number of] children. Therefore she said, "the Lord has given my reward since I gave my maidservant to my husband" (B'reishis 30:18). Additionally, Ya'akov would listen to her because of this, that he would establish many sons, for he knew this. This accords with the words of our rabbis (B'reishis Rabba 68:11).
And it may be that they followed their own reasoning, that since the land was given to their descendents, and Avraham and Yitzchak did not have many children, Ya'akov wanted many wives to increase the number of his children to inherit the land, for "the fourth generation would return here" (B'reishis 15:16), and therefore she wanted to give him her maidservant so that he would not resort to marrying a foreign woman.