Can I take a booking for a group of twenty people on my white label travel portal?

Partly, but a group that size usually has to be handled as a quote rather than one instant online booking. Live search results come from the suppliers connected to your portal, and most of them limit how many travellers you can search for in one go, because their published rates are built for individual and small family bookings. So your portal will happily handle a normal booking online, but once the party gets large the search will often come back empty or only show a few rooms or seats at a time.

What you can do is work the group through your portal in a different way. Your staff can split the party into several smaller bookings so everyone still ends up in the same system, or they can take the enquiry in, build a quote with the options they have found, and send that to the customer for approval. You can also add a group enquiry form to your portal so the request lands with your team instead of the customer hitting a dead end on the search page.

Be aware that proper group rates, with a deposit, a name list deadline and a release date, normally come from a contract you hold with the hotel or airline rather than from a live connection. Whether those terms can be loaded into your portal depends on the supplier and on what your plan covers, so it is not something we can promise for every case.

It is worth checking the traveller limit on each of your suppliers first, then deciding whether you want groups routed to an enquiry form or handled as split bookings by your staff.