The Castle can garrison a maximum of 300 Reinforcements simultaneously. This limit applies to Units stationed in the Castle and those en route.
For example, suppose one player sends 300 separate Reinforcements to your Castle. In that case, other players won't be able to send their troops to you while these Units are on their way. But once these Reinforcements reach your Castle, they will be considered one group. Therefore, they will occupy only one place within the limit.
If three players send you 100 separate Reinforcements each, other players won't be able to send Reinforcements until they arrive. Upon arrival, these Units will occupy 3 places within the limit — one for each player.
If you already have 200 Reinforcements in your Castle, other players can send only 100 more Unit groups.
If you have 300 Reinforcement groups in your Castle, nobody can send you more Reinforcements.