Charles Ellicott Commentary Exodus 12:4

"and if the household be too little for a lamb, then shall he and his neighbor next unto his house take one according to the number of the souls; according to every man`s eating ye shall make your count for the lamb." — Exodus 12:4 (ASV)

If the household be too little for the lamb. —There would be situations where the family would not be large enough to consume an entire lamb at one sitting. When this was the case, people were to join with their neighbors, either two small families joining together, or a large family sending some of its members to increase the numbers of a small one. According to Josephus (Bellum Judaicum, 6.9.3), ten was the least number regarded as sufficient, while twenty was not considered too many.

Every man according to his eating shall make your count for the lamb. —Rather, you shall count. In determining the number for any given Paschal meal, you shall “count men according to their eating,” admitting more or fewer, as they are likely to consume less or more.