Short answer. Just one. Rule 13, Section 2 provides that where several counsels appear for one party, that party is entitled to only one copy of any pleading, served upon the lead counsel if one is designated, or upon any one of them if no lead counsel has been designated.
What the law says
Where several counsels appear for one party, such party shall be entitled to only one copy of any pleading or paper to be served upon the lead counsel if one is designated, or upon any one of them if there is no designation of a lead counsel.
Rule 13, Section 2 — Filing and [s]ervice, defined. Read the full provision →
The default rule on service through counsel
Rule 13, Section 2 first establishes that once a party has appeared through counsel, service ordinarily runs through that counsel: if a party has appeared by counsel, service upon such party shall be made upon his or her counsel, unless service upon the party and the party's counsel is ordered by the court. That baseline rule assumes one counsel per party, which is why the rule then has to address what happens when a party has more than one lawyer, which is your situation.
What happens with two lawyers on one side
The rule answers this directly: where several counsels appear for one party, such party shall be entitled to only one copy of any pleading or paper to be served upon the lead counsel if one is designated, or upon any one of them if there is no designation of a lead counsel. So even though you have two lawyers, the opposing side only has to serve one copy. If a lead counsel has been designated in your case, that copy goes to the lead counsel; if no lead counsel has been designated, the opposing side may serve any one of your two lawyers.
The reverse situation, for comparison
The same section addresses the opposite scenario as well, where one lawyer represents several parties on the same side: where one counsel appears for several parties, such counsel shall only be entitled to one copy of any paper served by the opposite side. Both rules share the same underlying logic, that the number of copies served tracks the number of distinct parties or the number of distinct representational units in a practical sense, not simply the raw number of lawyers or clients involved on a given side of the case.
Why designating a lead counsel is worth doing
Because service on any one of your lawyers is valid when no lead counsel is designated, the practical coordination of who actually receives and acts on a served pleading is left to you and your two lawyers to sort out internally, rather than being guaranteed to reach a specific one of them. Designating a lead counsel removes that ambiguity, since the opposing side then has a clear, single point of service, and you can be confident about which of your lawyers is receiving copies of what the other side files.