The City can garrison a maximum of 300 Reinforcements simultaneously. This limit applies to Units stationed in the City and those en route.
For example, suppose one player sends 300 separate Reinforcements to your City. 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 City, 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 City, other players can send only 100 more Unit groups.
If you have 300 Reinforcement groups in your City, nobody can send you more Reinforcements.