One may place an eruv conditionally. For example, if one knows that on Shabbat two Torah scholars will be lecturing in two nearby towns, but he has yet to decide if he will attend the lecture to the east, the one to the west, or neither, he places one eruv in the east and one in the west and stipulates in his mind that on Shabbat he will decide which teĥum to use, the one based on his home, on the eruv in the west, or on the eruv in the east. Once he has made his decision, though, he is bound by his chosen teĥum. If he did not make a conscious decision but merely started walking in accordance with only one specific teĥum, he has thus established his teĥum and may no longer change his mind and walk in accordance with a different teĥum (Eruvin 82a; SA 413:1; MB ad loc. 8).
One may volunteer to place an eruv on behalf of all residents of a locale. He then declares: “With this eruv it shall be permitted for all local residents and their guests to walk 2,000 amot from this place in every direction.” Anyone who is aware of the existence of this eruv before Shabbat, even if he did not decide that the site of the eruv is his mekom shevita, may decide on Shabbat that he wishes to walk in that direction and may rely on the communal eruv. However, one who was unaware of the eruv’s existence before Shabbat may not rely on it (SA 413:1).
If there are so many local residents that the person placing the eruv cannot leave two meals’ worth of food for each one of them, then as long as he knows that there will be no more than twenty people who will want to rely on the eruv, he may place two meals’ worth of food for twenty people and declare: “With this eruv it shall be permissible for anyone who wishes to walk 2,000 amot from this place in every direction for all Shabbatot of the year.” This allows all who are interested in relying upon his eruv to do so, on condition that they are aware of its existence before the Shabbat they rely on it (SA 413:1; BHL s.v. “le-khol eĥad”). It seems that when it is clear that someone will make use of the eruv, the berakha should be recited, but when it is uncertain whether anyone will make use of it, no berakha should be recited.
If one places an eruv unconditionally, his teĥum is determined by the location of the eruv. He may not change his mind and follow the teĥum measured from his home.