As many as you want. You are only limited by the number of Orders. As long as the sum total of all the orders for all your accounts does not exceed 100,000 per month, it does not matter how many accounts or marketplaces you carry in one SellerLegend account.
The benefit of having multiple accounts/marketplaces in one single SellerLegend account is that you only need one set of credentials to access them all. Also, you can compare KPIs across accounts/marketplaces.