#define MAX_PER_PLAYER 2

new HousesPurchased[MAX_PLAYERS];

public OnPlayerConnect(playerid)
{
    HousesPurchashed[playerid] = 0;
    return 1;
}

//in the /buy command add
if(HousesPurchased[playerid] < MAX_PER_PLAYER)
{

}
else return SendClientMessage(playerid, -1, "You can only have 2 houses in total.");

//in the onplayerconnect, do a search through all houses to see if they match the owner name, then call:

HousesPurchased[playerid]++;

//If it >= 2, then set it to 2. You should be done then.